Microstructural and Mechanical Property of Ultra-Heavy Microalloyed Steel Plate Produced by Thermo Mechanical Control Process (TMCP)

Produced through the thermo-mechanical controlled process (TMCP), the ultra-heavy microalloyed steel plate exhibits significant potential for applications in ocean exploration. This study examines the effects of the TMCP parameters on the microstructure and mechanical properties across the full thickness of the plate, emphasizing the factors that contribute to strength and the mechanisms that govern toughness and plasticity. Consequently, it reveals a bimodal grain size distribution of ferrite near the surface and a normal distribution near the center. At 1/4 thickness, an appropriate cooling rate and the presence of precipitated phases facilitate the formation of acicular ferrite (AF), which increases the density of high-angle grain boundaries (HAGBs) and enhances crack deflection. Additionally, it is observed that carbon diffusion toward the center of the plate leads to the development of large martensite/austenite (M/A) islands and nano-sized precipitates. In the heavy plate, surface strength is primarily driven by dislocation strengthening (σd) and texture strengthening (σth), while the central regions depend on precipitation strengthening (σp) and solid-solution strengthening (σss). Toughness is not only related to the number of larger M/A islands that promote crack formation, but also to the density of HAGBs, which reduces the ability to hinder crack propagation. During in-situ tensile testing, slip bands and inhomogeneous deformation are initially observed on the ferrite side of phase boundaries. The slower growth of geometrically necessary dislocation (GND) density in fine ferrite grains effectively regulates the stress-strain distribution, contributing to the excellent ductility of steel.