This study presents tests concerned with welding thermal process-induced precipitation processes taking place in 10 mm thick steel S700MC subjected to the Thermo-Mechanical Control Process (TMCP) with accelerated cooling. The thermomechanical processing of steel S700MC leads to its refinement, structural defects and solutioning with hardening constituents. Tests of thin foils performed using a transmission electron microscope revealed that the hardening of steel S700MC was primarily caused by dispersive (Ti,Nb)(C,N) precipitates (being between several and less than twenty nanometers in size). In arc welding, depending on a welding method and linear energy, an increase in the base material in the weld is accompanied by the increased concentration of hardening microagents in the weld. The longer the time when the base material remains in the liquid state, the greater the amount of microagents dissolved in the matrix. During cooling, such microagents can precipitate again or remain in the solution. An increase in welding linear energy is accompanied by an increase in the content of hardening phases dissolved in the matrix and, during cooling, by their another uncontrolled precipitation in the form of numerous fine-dispersive (Ti,Nb)(C,N) precipitates of several nm in size, leading to a dislocation density increase triggered by type 2 internal stresses.
 A. Grajcar, K. Radwański., H. Krztoń , Microstructural analysis of a thermomechanically processed Si-Al TRIP steel characterized by EBSD and X-ray techniques, Solid State Phenomena 203-204, 34-37 (2013).
 A. Lisiecki, Welding of thermomechanically rolled fine-grain steel by different types of lasers, Archives of Metallurgy and Materials 59 (4), 1625-163 (2014).
 M. Opiela, Thermomechanical treatment of Ti-Nb-V-B microalloyed steel forgings, Materiali in Technologije 4, 587-591 (2014).
 T. Gladman, The physical metallurgy of microalloyed steels, The Institute of Materials, Cambridge University Press, Cambridge 1997.
 M. Opiela, Effect of thermomechanical processing of the microstructure and mechanical properties of Nb-Ti-V microalloyed steel, Journal of Materials Engineering and Performance 9, 3379-3388 (2014).
 J. Górka, Analysis of simulated welding thermal cycles S700MC using a thermal imaging camera, Advanced Material Research ISI Proceedings 837, 375-380 (2014).
 A. Grajcar, M. Różański, S. Stano, A. Kowalski, Microstructure characterization of laser-welded Nb-microalloyed silicon-aluminum TRIP steel, Journal of Materials Engineering and Performance 23 (9), 3400-3406 (2014).
 M. Żuk, J. Górka, A. Czupryński, M. Adamiak, Properties and structure of the weld joints of quench and tempered 4330V steel, Metalurgija 55 (4), 613-616 (2016).
 D. Janicki, Disk laser welding of armor steel, Archives of Metallurgy and Materials 59 (4), 1641-1646 (2014).
 J. Górka, Microstructure and properties of the high-temperature (HAZ) of thermo-mechanically treated S700MC high-yield- -strength steel, Materiali in tehnologije 50 (4), 617-621 (2016).
 K.S. Bang, W.Y. Kim, Estimation and prediction of HAZ softening in thermomechanically controlled - rolled and accelerated - cooled steel. Welding Journal 81, 174-179 (2002).
 J. Górka, Study of structural changes in S700MC steel thermomechanically treated under the influence of simulated welding thermal cycles, Indian Journal of Engineering and Materials Sciences 22, 497-502 (2015).