## 3 Results and Discussion
### Tensile Test Results  
Figure 10a shows the tensile strength and elongation of each specimen. From the TZ softening in the hardness profiles (Figures 2 and 6), the weld strength is predicted to increase and then saturate with increasing BM strength. The experimental results are in good agreement with the prediction, except for the MS1300 steel, which has a higher weld strength than expected. MS1300 welds have a smaller softened zone than the others, and it seems that a narrower TZ improves the weld strength. The joint efficiency, which is defined as the ratio of the tensile strength of the welds to that of the BM, decreased with increasing strength, whereas the elongation decreased (Table 3). Materials with an elongation of less than 5% were judged to have no ductility. Therefore, it can be confirmed that the welds  
Figure 8: Microstructure of the MS1300 steel. (F: ferrite; TM: tempered martensite; M: martensite). (**a**) Joint macrograph; (**b**) BM; (**c**) TZ; (**d**) PRZ; (**e**) RZ.  
Figure 9: Microstructure of the HPF1500 steel (PF: polygonal ferrite; TM: tempered martensite; M: martensite). (**a**) Joint macrograph; (**b**) BM; (**c**) TZ; (**d**) PRZ; (**e**) RZ.  
for DP590 and DP780 are ductile, whereas the welds for DP980, TRIP1180, MS1300, and HPF1500 are brittle. Figure 10b-g shows the results of the strain map measurement for each specimen obtained through the DIC method. In the case of the DP590 steel, stress concentration occurs in the BM, whereas the stress is concentrated in the PRZ and TZ for all the other steels. As the strength increases, the toughness and elongation decrease. Based on the strain map, the DP590 steel is observed to be broken at the base metal, and the other steels are broken in the PRZ and TZ owing to softening during welding.  
Figure 11 shows the fracture surfaces of the DP590, DP780, and DP980 steels. The area indicated by the white dotted lines represents the boundary of the shear lip. The shear lip area decreased as the strength increased. In the case of the DP590 steel (Figure 11a), necking and dimples were observed. In the case of the DP780 steel (Figure 11b), river patterns were observed on the fracture surface, and ductile fracture is confirmed on the fracture surface. The DP980 steel welds (Figure 11c) are brittle with almost no ductility, and a cleavage fracture is observed on the fracture surface. Through the analysis of the fracture surface, it can be confirmed that the DP590 and DP780 steels are ductile and that the DP980 steel is brittle, consistent with the tensile test results.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **DP590** & **DP780** & **DP980** & **TRIP1180** & **MS1300** & **HPF1500** \\ \hline Joint efficiency (\%) & 100 & 98.7 & 97.1 & 78.8 & 87.8 & 60.7 \\ \hline Elongation (\%) & 19.6 & 8.7 & 4.4 & 4.2 & 1.4 & 1.3 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Joint efficiency and elongation corresponding to the graph shown in Figure 10a.  
Figure 10: Tensile strength (**a**) and strain maps obtained using the digital image correlation (DIC): (**b**), DP590, (**c**) DP780, (**d**) DP980, (**e**) TRIP1180, (**f**) MS1300, and (**g**) HPF1500.