## 3 Results and Discussion
### TRIP1180, MS1300, and HPF1500 Steels  
Figure 6 shows the weldment hardness distributions for the TRIP1180, MS1300, and HPF1500 steels. The average hardness values of the TRIP1180, MS1300, and HPF1500 steel BM were 348, 447, and 500 Hv, respectively, and the average hardness values of the RZ were 475, 430, and 500 Hv, respectively. MS1300 steel has a lower hardness in the RZ than the TRIP1180 and HPF1500 steels, which can be attributed to the lower hardenability. The carbon equivalent (_Ceq_) for MS1300 is 0.27 wt%, whereas the _Ceq_ values for the TRIP1180 and HPF1500 steels are 0.32 wt% and 0.31 wt%, respectively, when using _Ceq_ (=C + Si/50 + Mn/25 + P/2 + Cr/25 + 14B) for high-strength steels obtained from a previous study [22]. The minimum hardness values in the PRZ and TZ were 295, 247, and 284 Hv for TRIP1180, MS1300, and HPF1500 steels, respectively, which indicates that the hardness was much lower than that of the BM due to softening. The peak temperatures of the PRZ and TZ are less than Ac\({}_{3}\); hence, martensite could be austenized and recrystallized only partially at most in the PRZ and transformed into tempered martensite in the TZ. Figure 7 shows the weld cross-section of the TRIP1180 steel welds by zone. The BM microstructure of the TRIP1180 steel comprises bainite, martensite, and retained austenite  
Figure 5: Microstructure of the DP980 steel (F: ferrite; TM: tempered martensite; M: martensite). (**a**) Joint macrograph; (**b**) BM; (**c**) TZ; (**d**) PRZ; (**e**) RZ. Figure 6: Hardness profiles for TRIP1180, MS1300, and HPF1500 steels. embedded in a primary ferrite matrix [23; 24]. TRIP steel transforms into martensite when stress is applied to the retained austenite in the BM microstructure, and these properties provide excellent strength and ductility. Therefore, the microstructure of the RZ shows a fully martensitic structure owing to the stress generated during the friction stir welding process, in addition to the high peak temperatures above Ac\({}_{3}\) and a rapid cooling rate (Figure 7e). As shown in Figure 7c, the TZ has a peak temperature lower than Ac\({}_{1}\); hence, the carbides located in the retained austenite are dispersed by tempering, and the dispersed carbides aggregate to form coarsened carbides. Softening occurs, and the hardness decreases owing to this phase transformation [24]. Figure 8 shows the weld cross-section of the MS1300 steel by region. The microstructure of the BM is fully martensitic (Figure 8b). The RZ has a martensitic structure with a finer packet than the BM because of rapid cooling after heating to a high temperature above Ac\({}_{3}\) (Figure 8e). Because the PRZ has a peak temperature between Ac\({}_{1}\) and Ac\({}_{3}\), martensite and tempered martensite can be simultaneously observed. In the TZ, the peak temperature is lower than Ac\({}_{1}\), and the martensite has transformed into heavily tempered structure, and ferrite structures can be observed (Figure 8c), which explains the severe softening in the PRZ and TZ. Figure 9 shows the weld cross-section of the HPF1500 steel by zone. HPF1500 steel is up to five times stronger than a conventional mild steel sheet, and it has a strength of 1500 MPa or more through martensitic quenching after heating above 900 \({}^{\circ}\)C, which is called hot press forming. Therefore, the microstructure of the BM exhibited a fully martensitic structure (Figure 9b). The RZ also has a fully martensite structure owing to the rapid cooling rates after being heated at a high temperature above Ac\({}_{3}\) (Figure 9e). In the PRZ, martensite and partially tempered martensite structures can be observed (Figure 9d). Because the peak temperature in the TZ is lower than Ac\({}_{1}\), martensite is transformed into tempered martensite or ferrite, and its structure has a mixture of polygonal ferrite and martensite (Figure 9c). This causes softening of the PRZ and TZ and lower hardness compared with that of the BM.