Abstract
The microstructure of fibre laser welded joint of TRIP 690T and HSLA LAD340 steel sheets with different thicknesses was evaluated. The microstructure in the fusion zone was built of martensite, lower bainite and a small portion of upper bainite. Martensite and lower bainite created in both the coarse-grained and fine-grained heat affected zones (HAZs) near 690T steel, but some portion of ferrite was also identified in the fine-grained HAZ near 690T steel. Acicular ferrite, upper bainite, and equiaxed ferrite were observed in both the coarse-grained and fine-grained HAZs near LAD340 steel, but lower bainite was occasionally found in the coarse-grained HAZ near LAD340 steel. Creation of hard microconstituents caused an increase in microhardness values in the fusion zone and both HAZs compared to the base metals. The highest microhardness of 457 HV_0.1 was measured in the coarse-grained HAZ near 690T steel with martensitic and lower bainitic microstructure.

Key words
TRIP steel, HSLA steel, fibre laser welding, microstructure

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