Using the thermodynamic model between TiN-BN-AlN in austenite, simulated heat-affected zone impact toughness of low-carbon type Ti-B containing steels was investigated. HAZ toughness was dependent on microstructure which was determined by the soluble boron to soluble nitrogen ratio at the welding peak temperature, 1350℃. Only when the soluble boron to soluble nitrogen ratio is kept below 0.2, HAZ showed ferrite dominant microstructure and had absorbed energy higher than 100J at - 20℃. It was shown that soluble boron combined with soluble nitrogen to 50rm boron nitrides during welding cooling cycle, resulting in a reduction of effective boron influencing the hardenability of HAZ. Compared to the conventional steels, low-carbon steels with a controlled soluble boron to soluble nitrogen ratio showed excellent HAZ toughness and no cold-cracking without preheating.