Effect of gadolinium on inclusions and mechanical properties of a high-sulfur free-cutting steel

The variation of inclusions composition in a high sulfur steel after the addition of gadolinium (Gd) were investigated to understand the influence of Gd on tensile and impact properties of the steel. With increasing the total gadolinium (T.Gd) content, the evolution path of inclusions was MnS/Al-Gd-O/Gd-O-S/(Gd-O-S)-(Gd-S). After the forging of the steel, long strip MnS inclusions exhibited the aggregated distribution, and spherical Gd-bearing inclusions were uniformly distributed in the steel. The proportion of inclusions with aspect ratio in the range of 1 ~ 3 increased from 25.7 % to 98.2 % when increasing T.Gd addition from 0 ppm to 462 ppm. After Gd addition in the steel, the tensile property of the steel was modified by transforming long strip sulfide inclusions into spherical ones and reducing the number density of inclusions. With increasing the T.Gd content from 0 ppm to 462 ppm, the tensile strength of samples increased from 402 MPa to 443 MPa. The grain size reached 19.4 μm and 13.0 μm in the steel with T.Gd contents of 150 ppm and 462 ppm, respectively. The impact property of the steel was influenced by inclusions size and grain size. The addition of gadolinium in the steel reduced the ductile-brittle transition temperature (DBTT) and improved the impact toughness of the steel at low temperature. The impact energy of the steel with 0 ppm and 462 ppm T.Gd at -60 oC were 8.6 J and 101.3 J, respectively. The steel sample with 462 ppm T.Gd containing exhibited the best comprehensive properties.