The influence of varying the manganese (Mn) contents of high-strength copper-containing hull steel on its microstructural evolution and mechanical properties was investigated. With increasing Mn content from 2 to 5%, the tensile strength of the steel increased by ~100 MPa, while the elongation of steel remained at ~23.5%, indicating good plasticity. However, the 2Mn sample had 128 J higher low-temperature (−84 °C) impact work than the 5Mn sample. The microstructures of different Mn steels were composed of fresh martensite (FM), ferrite/tempered martensite (F/TM), and reversed austenite (RA). The increase in Mn content markedly increased the presence of RA and intensified the work hardening caused by the transformation-induced plasticity (TRIP) effect during the tensile process. However, as the phase transformation in different Mn steels occurred in the early stage of strain and did not extend throughout the entire plastic deformation process, increasing plasticity via phase transformation was difficult. In addition, although the volume fraction of RA increased significantly in 4Mn and 5Mn steels, the stability of RA significantly decreased. The presence of numerous metastable blocks and coarse lath-like RA contributed little to low-temperature impact work and was even detrimental to toughness. The substantial fresh martensite resulting from phase transformation facilitated microcrack generation, owing to rapid volume expansion and mutual impacts, thus reducing the work required for crack formation. Additionally, the abundance of deformation twins significantly reduced the work needed for crack propagation. These combined actions significantly reduced the low-temperature toughness of 4Mn and 5Mn steels.

This study examined the effect of boron (B) on the microstructure and toughness of the simulated coarse-grained heat-affected zone (CGHAZ) of normalized vanadium microalloyed offshore steel by using the welding thermal simulation method under different heat inputs for welding. The results showed that when t_8/5 (the cooling time from 800 to 500 ${}^{\circ }$C) increased from 14 s to 24 s, In the range of t_8/5 of 24–44 s, the impact energy of the CGHAZ rose initially and then remained constant at around 125 J at −40 ${}^{\circ }$C, and dropped to 79 J when t_8/5 increased to 64 s. The particles (Ti,V)(C,N)-BN and BN contributed in the generation of acicular ferrite, which minimized the loss of CGHAZ toughness due to the presence of carbon. Furthermore, the microstructural parameters controlling CGHAZ toughness were the contents of the high misorientation grain boundaries and effective grain size at a tolerance angle of 15${}^{\circ }$ at varied heat inputs.

This study investigated the effects of reheating temperature on the microstructure and mechanical properties of Cu-containing 440 MPa grade non-tempered ship plate steel. The mechanical properties test, thermodynamic simulation, optical microscopy, scanning electron microscopy, transmission electron microscopy, and other tests were performed. The results revealed that with increasing reheating temperature, the ferrite grain size of Cu-containing 440 MPa non-tempered ship plate steel increased. Also, with increasing reheating temperature, the size of copper particles and niobium–titanium composite precipitates in the original austenite decreased. Consequently, this led to a weakening of the pinning effect on the original austenite and an increase in the size of the transformed ferrite grains. Moreover, with increasing reheating temperature, the number of Cu precipitates in the test steel after air cooling and rolling increased, while the size of the precipitates decreased, thereby weakening the solid solution strengthening effect of Cu, and precipitation was enhanced. Additionally, as the reheating temperature increased, the tensile strength and yield strength of the air-cooled test steel after rolling increased, while the impact toughness decreased.