Effect of prior rolling and thermal process on the occurrence of delamination in the tensile test and Charpy V-notch impact test specimens of Nb-microalloyed HSLA steel studied. Four different rolling and thermal process were carried out; hot rolled-quenched, hot rolled-air cooled, hot rolled-quenched-cold rolled, and hot rolled-air cooled cold rolled. the samples were aged at 600℃ for different period of times before tensile or impact test. The occurrence of the delamination was be depended on the fabrication process, aging time, and test temperature.
In the case of tensile specimens, the delamination occurred only in the hot rolled-quenched-cold rolled samples. In the case the delamination was initiated and propagated along grain boundaries. The susceptibility for the delamination at the grain boundaries is attributed to the elongated grains in the direction of cold rolling, to the precipitated carbides by aging treatment, and also to the nonmetallic inclusions such as MnS and $Al_2O_3$, at the boundaries.
In the initial stage of aging treatment, delamination increased with the increased amounts of crabides on the grain boundaries. The degree of delamination reached to a maximum value and then the decrease of delamination by increasing the aging time. The decrease of delamination can be interpreted as the result of the generation equiaxed grains by recrystallization and resulting the original carbides at grain boundaries to locate inside the new grains.
In the case of CVN impact test specimens, the delamination occurred in the hot rolled-quenched-cold rolled and hot rolled-air cooled-cold rolled samples. The fracture surface has shown transgranular brittle fracture mode.
By decreasing test temperature, the degree of delamination increased to a maximum value and then decreased rapidly. The temperature corresponding to the maximum degree of delamination was around the transition temperature range. With the increased of aging time, the degree of delamination of hot rolled-quenched-cold rolled samples decreased, however that of the hot rolled-air cooled-cold rolled samples kept a constant value after some decrease at the initial stage.
