# 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  
After testing the ASTM A572 Gr. 50 steel, the stress-strain curves at each target temperature were summarized, as shown in Figure 6. Figure 6a shows the stress-strain curve up to the initial 0.05 (5%) strain level. Using the stress-strain curve at the initial strain level, the diagram of the steel in the elastic section can be carefully observed. Figure 6b shows the stress-strain curve up to the 0.2 (20%) strain level. In general, it is common to obtain the strain of steel for design purposes of up to 20%; in this study, the range of the maximum strain was expanded to 20%. Based on the stress-strain curve of Gr. 50 steel for each target temperature, the difference in the stress according to the temperature was not large up to 300 \({}^{\circ}\)C, but the stress rapidly decreased above 400 \({}^{\circ}\)C. Above 400 \({}^{\circ}\)C, the strength of the steel material decreased significantly, while the difference in the room-temperature strength increased with an increase in the strain level. Overall, when the strain level of the maximum stress (tensile strength, \(f_{u}\)) is reached, the stress gradually decreases with an increase in the strain. At 800 \({}^{\circ}\)C or higher, even if the initial stress was low, the steel did not break, with a continuous increase in the strain of the steel material. The stress-strain curve \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Target** \\ **Temperature** \\ \end{tabular} } & \multicolumn{5}{c}{**Strength Value**} & \multicolumn{2}{c}{**Elastic**} \\  & & & \multicolumn{2}{c}{**(MPa)**} & & \multicolumn{2}{c}{**Modulus**} \\  & \multicolumn{2}{c}{} & & \multicolumn{2}{c}{**(MPa)**} & & \multicolumn{2}{c}{**(MPa)**} \\ \cline{2-7}  & **0.2\%** & **0.5\%** & **1.0\%** & **1.5\%** & **2.0\%** & **20\%** & **Max** & **Er** \\  & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{**(MPa)**} & \multicolumn{2}{c}{**(MPa)**} & \multicolumn{2}{c}{**(MPa)**} & \multicolumn{2}{c}{**(MPa)**} \\ \hline
25 & 332 & 292 & 358.7 & 397.3 & 418.7 & 466.7 & 514.7 & 205,000 \\  & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} \\ \((f_{y,25,0.2})\) & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} \\ \((f_{y,25,0.2})\) & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} \\ \((f_{y,25,0.2})\) & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} \\ \((f_{y,25})\) & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} \\ \hline
100 & 310.7 & 306.7 & 360 & 400 & 425.3 & 476.85 & 496 & 180,000 \\ \hline
200 & 321.3 & 318.7 & 373.3 & 412 & 437.7 & 502.65 & 516 & 182,000 \\ \hline
300 & 298.6 & 316 & 369.3 & 406.7 & 437.3 & 502.65 & 568 & 248,000 \\ \hline
400 & 268 & 276 & 320 & 350.7 & 374.7 & 405.3 & 460 & 220,000 \\ \hline
500 & 196.7 & 201.3 & 229.3 & 246.7 & 257.3 & 205.3 & 273.3 & 116,667 \\ \hline
600 & 128 & 138.7 & 152 & 156 & 160 & 133.3 & 162.7 & 85,000 \\ \hline
700 & 60 & 72 & 76 & _77.7_ & _77.7_ & 58.7 & _77.7_ & 50,000 \\ \hline
800 & 41 & 42.7 & 45.3 & 50.7 & 52 & 52 & 61.3 & 25,000 \\ \hline
900 & 28 & 28 & 32 & 33.3 & 33.3 & 34.7 & 40 & 16,667 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Strength values and elastic moduli for Gr. 60 at various strain levels obtained from the steady-state test. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Target** \\ **Temperature** \\ \end{tabular}