Nobutaka Kurosawa

Strengthening of ultra-low carbon steels by laser melting treatment

In order to reduce the weight of automobiles, the laser transformation hardening response of ultra-low carbon steels (0.003%C) was investigated by comparison with the response of low carbon steels (0.08%C). The effects of the nitrogen and titanium content on hardness are discussed. In the low carbon steels, the hardness of the weld metal showed little change with increases in the titanium content. However, in the ultra-low carbon steels, hardness increased with increasing titanium content. In the low carbon steels, the microstructures of the weld metal, which were comprised mainly of martensite, changed little with increasing titanium content. On the other hand, in the utlra-low carbon steels, the microstructures were bainitic ferrite, and the grains were refined with increasing titanium content. With increasing N2 content in the shielding gas, the hardness of the weld metal increased due to nitrogen absorption in the weld metal. Although the nitrogen contents of the ultra-low carbon steel welds were equivalent to those in the low carbon steel welds, the hardness increments of the former were larger than those of the latter. It was confirmed that increasing the N2 content of the shielding gas is beneficial for strengthening ultra-low carbon steels by laser transformation hardening.