Abstract: As the essential oxidizer of the third generation oxide metallurgy technology, Mg is helpful to improve the microstructure and toughness in coarse-grain heat affected zone (CGHAZ) of high strength low alloy steel (HSLA).The characteristic analyses of the inclusions,microstructure, and impact toughness in CGHAZ of HSLA steel of industrial experiment with Mg deoxidation were conducted by using inclusion automatic analysis system, optical microscope and welding simulator.The results showed that under the welding with high heat input of 200 kJ/cm, a large amount of Mg-Ti-Mn-S inclusions are generated in the CGHAZ of Mg deoxidized steel. Due to this type of inclusion promoting the formation of intragranular acicular ferrite (IAF), after the welding with high heat input of 200 kJ/cm, the CGHAZ of Mg deoxidized steel exhibits excellent low-temperature impact toughness, with an impact energy of 204 J at -20 ℃, and the ductile fracture. The results of this study can provide theoretical and experimental basis for improving the microstructure and properties of steel plate for high heat input welding.

Key words: HSLA steel, Mg deoxidation, high heat input welding, heat affected zone, low-temperature impact toughness