## 3 Results and Discussion
### Welding Test Result  
Full penetration was achieved under bead-on-plate welding conditions, and the higher the welding speed, the narrower the bead width. Sound weld beads were formed except for a slight underfill (Figure 5). In the cross-sectional images, white bands were observed in the HAZ between the WM and BM. According to a previous study [12], that white band region indicated the ICHAZ, which had a peak temperature between the austenite transformation temperatures of A\({}_{\text{C1}}\) and A\({}_{\text{C3}}\). The ICHAZ of laser-welded HPF steel contains relatively soft ferrite owing to partial austenization, while other regions comprise martensite and tempered martensite. The microstructure of the weld specimen with a welding speed of 7 m/min was analyzed using OM and SEM. The WM has a microstructure of lath martensite owing to the high cooling rate of laser welding (Figure 6a), which is similar to that of the BM (Figure 4a). A previous study on laser welding on 1.5 GPa HFP steel reported that the WM of 1.5 GPa HFP steel has a finer martensitic microstructure with a smaller packet size than the BM [15]. Although the cooling rate during laser welding is much faster than that during the HPF process [16; 17], the BM of 2.0 GPa HPF was almost fully hardened through the HPF process owing to the high carbon content, which resulted in a similar microstructure in both the BM and WM. The upper critical HAZ (UCHAZ), where the peak temperature is between the A\({}_{\text{C3}}\) and the melting temperatures, is composed of coarse grain HAZ (CGHAZ) adjacent to the WM and fine grain HAZ (FGHAZ) adjacent to the ICHAZ. Both CGHAZ and FGHAZ showed a martensitic structure owing to the high cooling rate. The martensite packet in the FGHAZ is slightly finer than that in the CGHAZ, but the microstructural difference is not significant (Figure 6b,c): the microstructure of the  
Figure 4: Characteristics of base material: (**a**) SEM image of the base metal (5000\(\times\) magnification) and (**b**) tensile test result. Figure 5: Bead-on-plate weld cross-sections according to welding speed (45\(\times\) magnification): (**a**) 6 m/min; (**b**) 7 m/min; (**c**) 8 m/min. ICHAZ differs from that of other regions. In the ICHAZ, martensite was formed in the prior-austenite grain boundary, and ferrite (or ferrite/carbide) was formed in the rest of the areas (Figure 6d). Ferrite has different polishing and etching characteristics from those of martensite and tempered martensite, which resulted in white bands on the cross-sections in Figure 5. In the SCHAZ, the austenite phase transformation did not occur because the peak temperature was below the A\({}_{\text{C1}}\) temperature, and tempered martensite with carbide precipitates was observed (Figure 6e). Compared to the BM, the WM and UCHAZ had similar or higher hardness, while lower hardness was measured in the ICHAZ and SCHAZ (Figure 7). As the welding speed increased from 6 to 8 m/min, the width of the high-hardness zone of the WM and UCHAZ decreased. The softening of the ICHAZ and SCHAZ was caused by the formation of a relatively soft microstructure of ferrite and tempered martensite, as shown in Figure 6.  
Nanoindentation hardness testing was carried out in the lower-hardness area of the ICHAZ and SCHAZ on the specimen with a welding speed of 7 m/min. Hardness was measured at 13 locations along a line oblique to the HAZ boundaries (Figure 8a). The lowest hardness value of 323.7 HV was observed at location 7 (Figure 8b), which was placed on the exact boundary between the ICHAZ and SCHAZ as shown in the SEM image (Figure 8a). Ferrite and tempered martensite were simultaneously observed in the microstructure of the lowest-hardness zone (Figure 9). It was confirmed that the most softened location was the ICHAZ/SCHAZ boundary where the peak temperature was equal to the A\({}_{\text{C1}}\) temperature.