Particles,Microstructures, and Impact Toughness of CGHAZ of Ca Deoxidation Shipbuilding Steel Plates with Different Nb Contents

Herein, the effects of the Nb content on the particles, microstructure evolution, and mechanical properties of base metal (BM) and high-heat input coarse-grained heat-affected zone (CGHAZ) of shipbuilding steel plates with Ca deoxidation are studied. With increasing Nb content, the strength and microhardness of the BM increase without significant loss of impact toughness. However, in CGHAZ, the average grain size increases from 168 to 244 μm, and the brittle phase fraction increases from 0.33% to 11.76%. The high-angled grain boundary (HAGB) density in microstructures reduces from 0.35 to 0.19 μm⁻¹, and the average effective grain sizes increase from 10.97 to 12.48 μm. The particles in 40 Nb-BM and 40 Nb-CGHAZ are mainly cubic TiN particles. In 170 Nb, except for cubic particles, there are approximately ellipsoid particles with high value of Nb content, which tend to dissolve into matrix at the elevated temperature. With increasing Nb content, the average size of particles in CGHAZ increases from 18.33 to 32.29 nm, and the number density decreases from 13.44 to 10.35 μm⁻². Therefore, the impact toughness of CGHAZ decreases from 197 to 96 J at −20 °C under the high-heat input welding of 400 kJ cm⁻¹.