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Distinct roles of Nb, Ti, and V microalloying elements on the fire resistance of low-Mo steels

2025-06-30T15:00:00Z

Title: Distinct roles of Nb, Ti, and V microalloying elements on the fire resistance of low-Mo steels Author(s): Park, Hyungkwon; Jo, Hyo-Haeng; Kim, Seong Hoon; Kim, Chiwon; Moon, Joonoh; Chung, Jun-Ho; Lee, Bong-Ho; Lee, Chang-Hoon Abstract: The high-temperature mechanical stability of structural steels is critical for fire-resistant applications, yet the individual contributions of microalloying elements remain incompletely understood. In this study, we systematically investigated the strengthening mechanisms of three key microalloying elements-Nb, Ti, and V-in low-Mo fire-resistant steel. Each element was independently added to a 0.15Mo-base steel at varying concentrations, and tensile tests were conducted at room temperature (RT) and 600 degrees C. Microstructural features were characterized in detail using atom probe tomography. Ti enhanced the yield strength (YS) at both temperatures via the formation of (Ti,Mo)(C,N) precipitates, but excessive Ti reduced the YS ratio (6600oC/6RT) due to solute depletion and precipitate coarsening. V demonstrated minimal precipitation and limited impact at RT, but its linear contribution to high-temperature strength is attributed to secondary hardening by VC. Nb yielded the most consistent strengthening across both temperatures through the combined effects of (Nb,Mo)C precipitation, Nb-C clustering, solid solution strengthening, and bainitic transformation. These findings clarify the element-specific mechanisms governing fire-resistant behavior and suggest that optimized microalloying strategies can enable steels with superior strength retention at elevated temperatures.

Microstructural evolution and strength-toughness behavior of fire-resistant steel under thermo-mechanical controlled processing

2025-06-30T15:00:00Z

Title: Microstructural evolution and strength-toughness behavior of fire-resistant steel under thermo-mechanical controlled processing Author(s): Park, Hyungkwon; Jo, Hyo-Haeng; Kim, Chiwon; Kim, Seong Hoon; Kim, Kyeong-Won; Moon, Joonoh; Hong, Hyun-Uk; Chung, Jun-Ho; Lee, Bong-Ho; Lee, Chang-Hoon Abstract: Fire-resistant steels are engineered to retain high strength at elevated temperatures, typically maintaining a yield strength (YS) ratio (600 degrees C/RT) above 0.67 to prevent sudden structural collapse during fire exposure. Although achieving such strength retention is critical, ensuring sufficient fracture toughness is equally essential for structural reliability. However, toughness behavior in fire-resistant steels has been largely overlooked, particularly in relation to thermomechanical processing. In this study, the influence of thermomechanical controlled processing (TMCP) on the microstructure and mechanical behavior of fire-resistant steel was investigated, with an emphasis on the strength-toughness trade-off. With increasing TMCP conducted below the nonrecrystallization temperature (Tnr), the bainite fraction decreased markedly from 79 % to 27 %, whereas the ferrite fraction increased. The prior austenite grain size of the bainite was significantly refined, whereas the ferrite grain size remained nearly unchanged. This microstructural evolution led to a gradual reduction in yield strength (YS) at both room and elevated temperatures, decreasing the YS ratio (600 degrees C/RT) from 0.717 to 0.501. Meanwhile, the Charpy impact energy increased from 32.9 to 169.5 J, thereby demonstrating a clear trade-off between strength and toughness. Notably, the bainite fraction exhibited a strong linear correlation with the strength and YS ratio, whereas ferrite played a dominant role in enhancing toughness, with a complementary contribution from bainitic grain refinement. These findings demonstrate that the mechanical performance of fireresistant steels can be effectively tuned through process optimization alone, thereby providing a practical strategy for designing steels with balanced strength and toughness.

Surface hardening of ductile austenitic lightweight steel through powder bed fusion 3D printing

2025-04-30T15:00:00Z

Title: Surface hardening of ductile austenitic lightweight steel through powder bed fusion 3D printing Author(s): Moon, Joonoh; Hong, Hyun-Uk; Park, Hyungkwon; Jo, Hyo-Haeng; Park, Seong-Jun; Shin, Chansun; Han, Heung Nam; Lee, Myoung-Gyu; Jeong, Jae Suk; Lee, Bong-Ho; Lee, Chang-Hoon Abstract: To save energy and reduce CO2 emissions, the lightweight design of structural components has recently become a global issue. Fe-Mn-Al-C based alloys with a low mass density have received considerable attention as structural materials enabling such lightweight designs. However, typical strength-ductility trade-off dilemma appears in Fe-Mn-Al-C lightweight steels. Dispersion of nano-sized Fe3AlC-type kappa-carbides achieves excellent tensile properties of high strength (similar to 1 GPa) and large elongation (similar to 50 %). However, further increase in strength (similar to 1.2 GPa) caused by kappa-carbide coarsening reduces elongation significantly (<10 %), limiting the potential applications of lightweight steels in structural parts that require ultrahigh strength and high ductility, such as wear-resistant components. Here, we resolve this drawback of lightweight steels by reinforcing the surface layer through 3D printing. The composition of base steel plate is Fe-30Mn-8Al-0.7C (wt%), and a lightweight steel powder with a relatively higher Al and C contents (Fe-30Mn-9.5Al-1.0C (wt%)) was then deposited on the surface of base steel plate through laser powder bed fusion (L-PBF). After L-PBF, an aging treatment led to more precipitation of kappa-carbides in the surface layer, producing a functionally graded hard surface layer. A developed surface-hardened ductile lightweight steel thus has the potential to replace conventional wear-resistant steels, as it has excellent tensile ductility (51 %), high surface hardness (410 HV), high wear resistance, and 12 % lower mass density.

Mutual strengthening by fine MX precipitation and solute segregation on dislocations during complex low-cycle fatigue to simulate the seismic/fire situation in bainitic H-section steel

2025-10-31T15:00:00Z

Title: Mutual strengthening by fine MX precipitation and solute segregation on dislocations during complex low-cycle fatigue to simulate the seismic/fire situation in bainitic H-section steel Author(s): Han, Jae-Yeon; Park, Jung-Hyeon; Yang, Cheol-Hyeok; Lee, Bong Ho; Park, Hyungkwon; Lee, Chang-Hoon; Chung, Jun-Ho; Moon, Joonoh; Hong, Hyun-Uk Abstract: In this study, a low carbon bainitic H-section steel alloyed finely with Mo, V, Ti, and Nb was developed for applications requiring both seismic and fire resistance. To assess its structural integrity following earthquake and fire event, a combination of room-temperature low-cycle fatigue (LCF) testing up to 10 cycles (to simulate earthquake conditions) and thermal exposure at 600 °C for 2 h (to simulate post-earthquake fire scenario) was conducted. It was interesting that the specimens, which underwent the aforementioned complex LCF testing combined with thermal exposure, exhibited a fatigue life comparable to that observed after a single LCF test. Fine MX precipitates additionally precipitated during thermal exposure. Furthermore, atom probe tomography results indicated that the uniform formation of nano-sized (C,V,Mo,Nb)-rich clusters were discovered after thermal exposure, and their segregation along dislocation cores was found, forming Cottrell atmosphere. These strong hindering/dragging effects of excess solute atoms on dislocations facilitated the occurrence of serrated flow on the hysteresis loop during the 11th cycle immediately after thermal exposure. These results suggest that the mutual strengthening resulting from fine additional MX precipitation, solute clustering, and segregation on dislocations primarily contributes to the suppression of cyclic softening, ultimately leading to an excellent complex LCF resistance. © 2025 Elsevier Ltd