4. Conclusions
A detailed microstructure characterization of different weld
regions of developed Nb-microalloyed TRIP-type steel with
silicon partly replaced by aluminum is included. The result of
the process, conducted in an air atmosphere, is the presence of
numerous non-metallic inclusions of various sizes in the fusion
zone. On the other hand, it is necessary to note that using a
shielding gas significantly decreases the efficiency of laser
welding and increases costs. The use of a remote welding
technique, which is becoming increasingly more widespread in
the automotive industry, would require the application of a
shielding gas blowing system integrated with fitting accessories.
Satisfactory weld quality is confirmed by the lack of HAZ
softening, microcracks, and solidification cracking, which pose
frequent problems during welding of thermomechanically
rolled steel sheets. Further work is needed to assess weldability
of the investigated steel. The base metal is characterized by a
fine-grained ferritic-bainitic microstructure with a 15% fraction
of retained austenite in the form of blocky grains and interlath
retained austenite. A high cooling rate of the laser-welded joint
fully determines the steel microstructure in the fusion zone and
in the heat-affected zone. It is composed of lath martensiticbainitic
constituents containing several percent of interlath
retained austenite. Due to carbon enrichment of the austenite,
the fraction of the c-phase in the intercritical HAZ is close to its
amount in the base metal. The welding linear energy, between
0.037 and 0.048 kJ/mm, has no effect on the microstructure and
microhardness of steel, which shows the highest values up to
510 HV0.1 in the fine-grained HAZ