Process Routes of Low‐Ni Liquefied Natural Gas Tank Steel with Excellent Cryogenic Toughness

Herein, several treatments, including modified quenching and tempering (QT) treatment, ultrafast cooling and tempering (UFC‐T) treatment, and ultrafast cooling, intercritical quenching, and tempering (UFC‐LT) treatment, are used to prepare low‐Ni steels and compared with conventional QT treatment. All the three treatments enable low‐Ni steel to achieve equivalent or better cryogenic toughness than 9% Ni steel. The toughening effect of modified QT treatment is better than that of UFC‐T treatment, which is attributed to the increase in density of high‐angle grain boundaries (HAGBs). For modified QT treatment, fine equiaxed prior austenite grains and weak variant selection of martensite transformation lead to higher density HAGBs. Although the coarse elongated prior austenite grains in UFC‐T treatment are beneficial to Bain grouping and not conducive to matrix refinement, the intercritical quenching step in UFC‐LT treatment further increases the density of HAGBs due to the jagged prior austenite grain boundaries (PAGBs) and fresh martensite formed along PAGBs. Therefore, UFC‐LT treatment achieves high‐density HAGBs and excellent cryogenic toughness similar to modified QT treatment. Due to the wider tempering temperature process window, UFC‐LT treatment is the optimal process route for low‐Ni liquefied natural gas tank steel.