Coarse‐Grain Heat‐Affected Zone Microstructure Evolution and Crack Propagation of Ca‐Treated Q550 Thick‐Steel Plate by High‐Heat Input Welding

The traditional research believes that the inclusion size of Ca‐treated steel is large and the mechanical properties are not good, so it is rarely used in high‐heat input welding. Although there have been reports in recent years about Ca‐treated steel for high‐heat input welding, the study on its evolution process and mechanism is very lacking. In this article, the evolution of coarse grain heat‐affected zone of Ca‐treated steel was studied by in‐situ observation, microstructure analysis, and low‐temperature impact test. The results indicate that Ca–Al–O inclusions, with a size of about 7 μm, effectively prevent austenite grain boundary migration and crack propagation while promoting the formation of lath bainite. During the cooling process, small‐sized inclusions precipitated from the test steel also facilitate the nucleation of intracrystalline lath structures. Unlike traditional Ti, Mg, and other deoxidized welding steels, when the heat input increases from 200 to 400 kJ cm −1 , the microstructure type of the test steel is still ferrite + lath‐bainite, but the number of lath‐bainite near the grain boundary decreases, and the grain misorientation decreases significantly. Grain boundary, lath structure, and M/A effectively prevent crack propagation, but the hindering effect decreases successively.