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800 MPa grade new ultra-low carbon bainitic (NULCB) steel is the recently developed new generation steel. The microstructure in the coarse-grained heat-affected zone (CGHAZ) of NULCB steel under laser welding conditions was investigated by thermal simulation. The influence of the cooling time from 800 ℃ to 500 ℃, t8/5 (0.3~30 s), on the microstructure of the CGHAZ was discussed. The experimental results indicate that the microstructure of the CGHAZ is only the granular bainite which consists of bainitic ferrite (BF) lath and M-A constituent while t8/5 is 0.3~30 s. The M-A constituent consists of twinned martensite and residual austenite, and the change of the volume fraction of the residual austenite in the M-A constituent is very small when t8/5 is between 0.3 and 30 s. The morphology of the M-A constituent obviously changes with the variation of t8/5. As t8/5 increases, the average width, gross and shape parameter of the M-A constituent increase, while the line density of the M-A constituent decreases.
800 MPa grade new ultra-low carbon bainitic (NULCB) steel is the recently developed new generation steel. The microstructure in the coarse-grained heat-affected zone (CGHAZ) of NULCB steel under laser welding conditions was investigated by thermal simulation. The influence of the cooling time from 800 ° C to 500 ° C, t8 / 5 (0.3-30 s), on the microstructure of the CGHAZ was discussed. The experimental results indicate that the microstructure of the CGHAZ is only the granular bainite which consists of bainitic ferrite (BF) lath and MA constituent while t8 / 5 is 0.3 to 30 s. The MA constituent consists of twinned martensite and residual austenite, and the change of the volume fraction of the residue austenite in the MA constituent is very small when t8 / 5 is between 0.3 and 30 s. The morphology of the MA consisting obviously changes with the variation of t8 / 5. As t8 / 5 increases, the average width, gross and shape parameter of the MA constituent increase, while the line density of the MA constituent decreases.