## 3 Results and Discussion  
### DP590, 780, and 980 Steels  
The weld cross-section of the steel FSW exhibited a heterogeneous microstructure. In this study, austenization phase transformation occurred in the welds because the peak temperature was higher than the Ac\({}_{3}\) temperature under all conditions. The Ac\({}_{1}\) temperature represents the temperature at which pearlite begins to austenite during heating and the Ac\({}_{3}\) temperature represents the temperature at which free ferrite is completely transformed into austenite during heating. The welds can be classified into four regions: (1) recrystallized zone (RZ), which comprises a stir zone and a heat-affected zone where the peak temperature is higher than the Ac\({}_{3}\) temperature. The entire region was fully austenized when the temperature rose and then recrystallized because of phase transformation when cooled; (2) partial recrystallized zone (PRZ): the peak temperature is between Ac\({}_{1}\) and Ac\({}_{3}\); hence, the region cooled down after partial austenization at the peak temperature; (3) tempered zone (TZ): Since the peak temperature is lower than Ac\({}_{1}\), the initial microstructure is not transformed to austenite, and only grain growth and tempered \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Tool Rotation** \\ **Speed (rev/min)** \\ \end{tabular} } & **Welding Speed** \\  & **(mm/s)** & **Angle (\({}^{\circ}\))** \\ \hline \multirow{2}{*}{600} & \multirow{2}{*}{4} & \multirow{2}{*}{2} & \multirow{2}{*}{10} & \multirow{2}{*}{2.5} & \multirow{2}{*}{0.85} \\  & & & & \\ \end{tabular}
\end{table} 
Table 2: FSW conditions and applied tool configuration. Figure 1: Schematics of (**a**) FSW process, (**b**) tool shape used in FSW process (unit: mm). martensite could be observed because of the tempering effect; and (4) base metal (BM): the initial microstructure is maintained. Each zone was identified using the microstructure, and also the peak temperature was estimated. The average hardness values of the DP590, DP780, and DP980 steels in the RZ were 310, 332, and 494 Hv, respectively, which were significantly higher than those in the other zones (Figure 2). The average hardness values of the BM of DP590, DP780, and DP980 steels were 227, 294, and 330 Hv, respectively, and the hardness increase in the RZ relative to the BM was approximately 37% for DP590, 13% for DP780, and 50% for DP980. The hardness increase in the RZ is relatively high in the DP980 steel compared with the other DP steels. This is because the DP980 steel has high hardenability owing to its relatively high carbon content compared with the other DP steels, as seen from the chemical compositions listed in Table 1. Unlike DP590, the DP780 and DP980 steels underwent softening in the PRZ and TZ, and the minimum hardness values in these zones were 265 Hv and 257 Hv, respectively, which are lower than the BM hardness. Slight softening occurred in the DP780 steel, whereas severe softening occurred in the DP980 steel. The softening of DP980 steel results from tempered martensite, and the high-volume fraction of martensite in the BM caused severe softening in the PRZ and TZ. Figure 3 shows the weld cross-section of the DP590 steel welds by zone. The microstructure of the BM of the DP590 steel comprises a ferrite matrix and martensite islands along the matrix boundary (Figure 3b). In the RZ, as shown in Figure 3e, the ferrite grain size decreased, and the martensite fraction increased. As a result, the hardness of the RZ is higher than that of the BM. Because the PRZ has a peak temperature between Ac\({}_{1}\) and Ac\({}_{3}\), the initial microstructure is partially transformed into austenite during heating, and when cooled, fine ferrite grains and martensite are formed in the region where partial transformation occurred previously (Figure 3d). The PRZ has a lower volume fraction of martensite than the RZ because only partial austenization occurred, and its cooling rate was lower than that in the RZ.