<p>The present study investigates the microstructure and mechanical properties of butt joints in a (3+16) mm thick S31603/Q500qENH composite steel plate under both flat and vertical welding conditions. The phase diagram method was utilized to derive the continuous cooling transformation (CCT) curves for Q500qENH steel and welding materials. The energy-cooling time relationship, as derived from the Rykalin 2D model of heat conduction, was utilized to propose the optimal heat input range for welding the base metal and transition layer of the S31603/Q500qENH composite steel plate. The findings suggest that decreasing the velocity of welding in the vertical direction results in elevated levels of heat input, consequently leading to diminished overall mechanical performance of the joint when compared to flat welding. During the welding of the transition layer in the composite steel plate, excessively high heat input increases the migration of C, Cr, and Ni between the transition-layer weld and the Q500qENH steel weld, leading to the formation of unfavorable phases, such as Cr₂₃C₆ carbides and the brittle σ phase. Conversely, insufficient heat input has been shown to result in a substantial increase in the cooling rate, thereby increasing the probability of martensite formation. In order to circumvent the formation of brittle phases and enhance the toughness of welded joints, this study integrates welding experiments, CCT curves, and an energy-cooling time relationship model to propose an optimal range of welding heat inputs for S31603/Q500qENH composite steel plates. The findings provide essential theoretical guidance and practical recommendations for the optimization of welding processes for composite steel plates.</p>