Abstract: Extensive efforts have been made to remove “harmful” inclusions during the steelmaking process. However, the concept of “oxide metallurgy” was proposed, where fine inclusions are used to induce the formation of acicular ferrite and pin the grain boundary, thus enhancing the low temperature toughness of the heat-affected zone (HAZ). The technology of improving the toughness of HAZ by forming TiO_x−MgO−CaO fine particles (ITFFP) in steel has been successfully applied to the trial production of 30 mm (H30) and 60 mm (H60) thick high heat input welding EH420 offshore steel. The mechanical testing results show that the yield strength, tensile strength, and elongation of H30 steel are 461 MPa, 579 MPa, and 26%, respectively. In addition, the yield strength, tensile strength, and elongation of H60 steel are 534 MPa, 628 MPa, and 24.5%, respectively. The tested H30 and H60 steels achieved the national standard of EH420 offshore steel. The effect of ITFFP technology on the microstructure and impact toughness in the HAZ of H30 and H60 steels subjected to a 200 kJ·cm⁻¹ heat input were investigated using a Gleeble-3800 welding simulation machine and Charpy impact tests. The results indicate that the CaO(−MgO)−Al₂O₃−TiO_x−MnS formed in the tested steels induces the formation of acicular ferrite, and thus significantly improves the impact toughness. Additionally, electrode-gas welding with heat inputs of 247 kJ·cm⁻¹ and 224 kJ·cm⁻¹ was applied to H30 and H60 steels. The experimental results show that the impact absorbed energy of the weld in H30 tested steel is larger than 74 J at −40 ℃, and the HAZ exhibits an absorbed energy larger than 115 J at −40 ℃. In addition, the impact absorbed energy of the weld in H60 tested steel is larger than 91 J at −40 ℃, and the HAZ exhibits an absorbed energy larger than 75 J at −40 ℃. The impact absorbed energy of welded joints is much higher than the requirement of the national standard (42 J).