Effect of Boron in Migitating Softening and Enhancing Mechanical Properties of HAZ in Low‐Carbon Martensitic Steel

In this article, the mechanism of boron in mitigating the heat‐affected zone (HAZ) softening in hot‐rolled ultra‐low‐carbon (C < 0.07 wt%) martensitic steels was investigated. The thermal cycles corresponding to different HAZs were simulated for 1000 MS steels with low (5 ppm) and high (12 ppm) boron contents. Results indicated that welding thermal cycles induced varying degrees of softening and strength reduction across all HAZs. The addition of 12 ppm boron effectively suppressed softening in both the fine‐grained zone (FG) and intercritical zone, increasing their hardness by 79 and 59 HV and yield strength by 262 and 219 MPa, respectively. This enhancement is attributed to boron segregation at prior‐austenite grain boundaries, which hinders carbon diffusion, prevents the formation of carbon‐depleted zones, and consequently inhibits ferrite precipitation, promoting martensite formation during the cooling stage. Furthermore, boron addition altered the softening hierarchy, shifting the most severely softened region from the FG (336HV) to the sub‐critical zone (SC, 288HV).