Morphology and Crystallography Analyses of HSLA Steels with Hardenability Enhanced by Tailored C–Ni Collocation

Abstract

High hardenability is of great importance to ultra-heavy steel plates and can be achieved by tailoring the composition of steel. In this study, the continuous cooling transformation (CCT) curves of two high-strength low-alloy (HSLA) steels (0.16C-0.92Ni steel and 0.12C-1.86Ni steel) were elucidated to reveal the significance of C–Ni collocation on hardenability from the perspective of morphology and crystallography. At a low cooling rate (0.5 °C/s), the 0.12C-1.86Ni steel showed higher microhardness than 0.16C-0.92Ni steel. The microstructure in 0.16C-0.92Ni steel was mainly granular bainite with block-shaped martensite/austenite islands (M/A islands), while that in 0.12C-1.86Ni steel was typically lath bainite with film-shaped M/A islands, denoting that the 0.12C-1.86Ni steel is of higher hardenability. Moreover, the 0.12C-1.86Ni steel exhibited a higher density of block boundaries, especially V1/V2 boundaries. The higher density of block boundaries resulted from the weakened variant selection due to the larger transformation driving force and more self-accommodation of transformation strain induced by the reduced carbon and increased nickel content.

Unraveling the Mechanism of Toughness Fluctuation in Ultra-High-Strength Casing from the Perspective of Crystallography

“…The For better comprehension of the relationship between intervariant boundaries and impact toughness, the intervariant boundaries were further categorized into block, subblock and packet boundaries according to the boundary types [13,15], and the densities of the intervariant boundaries were calculated. As shown in Figure 6a, the head exhibits a significantly higher density of block boundaries from different Bain groups than the end, which is a result of its weaker variant selection [8,36]. Previous studies have shown that a lower starting temperature of transformation can be triggered by a higher cooling rate, and thus a greater driving force for phase transformation can be obtained [8,35,37].…”