Effect of Ca Addition on Inclusions, Microstructures, and Impact Toughness of Coarse‐Grained Heat‐Affected Zone After High‐Heat Input Welding in Mg‐Deoxidized Shipbuilding Steel Plates

Effect of Ca addition on inclusions, microstructures, and impact toughness of coarse‐grained heat‐affected zone (CGHAZ) in Mg‐deoxidized shipbuilding steel plates after high‐heat input welding (HHIW) of 400 kJ cm −1 are investigated. Characteristics of inclusions and their effect on microstructures and impact toughness are elucidated. With the addition of Ca, number density of inclusions decreases from 384 to 273 mm −2 , but average size increases from 1.23 to 1.32 μm. Number densities of inclusions such as MgAl 2 O 4 , MgAl 2 O 4 + MnS, Mg–Al–Ti–O + MnS, MgO + MnS, and TiN + MnS, which are observed to induce intragranular acicular ferrites (IAFs) formation, decrease from 3.0, 15.3, 18.8, 16.5, and 18.1 to 0.2, 1.2, 4.4, 3.0, and 6.3 mm −2 , respectively. Observed Ca‐containing inclusions of diameter over 2.5 μm in Mg–Ca steel cannot serve as the effective nucleation sites for IAFs formation. Contents of IAFs and polygonal ferrites decrease from 73.20% and 19.96% to 54.34% and 5.63%, respectively. Frequency of high‐angle grain boundaries decreases from 70.5% to 51.3%. The CGHAZ fracture morphology changes from ductile fracture to brittle fracture with the area proportion of the fibrous zone decreasing from 56.5% to 0%. Hence, the impact toughness of CGHAZ decreases significantly from 175 to 23 J at −20 °C after HHIW.