Influential factors on the deformability of colonies in pearlitic steel

Highlights: • Strain distribution introduced by deformation in pearlitic steel was investigated. • Strain is distributed inhomogeneously as colony units. • Correlation between morphology of cementite and deformability has been elucidated. • Primary factor affecting the deformability of colonies is the cementite length. • Other influential factors on the deformability of colonies were also discussed. The inhomogeneous strain distribution introduced by the tensile deformation of pearlitic steel was visualized using digital image correlation method, and the morphology of cementite was quantitatively evaluated. The high strain was introduced near the block boundaries. Within the blocks, the strain was inhomogeneously distributed. The ratio (L_cem/L_col) of cementite length to colony size, which is related to the degree of spheroidization, was well correlated with the average strain within the colonies; spheroidized colonies exhibited a relatively high strain. However, although the L_cem/L_col values were nearly identical, the values of the introduced strain were largely dispersed in the colonies. Based on the L_cem/L_col values, the colonies were classified as spheroidized, partially-spheroidized, and lamellar colonies. In majority of the lamellar colonies, the introduced strain was lower than the average strain and tended to be higher in the colonies located near the block boundaries. On the other hand, for the partially-spheroidized colonies, the values of the introduced strain varied considerably and were sufficiently correlated with the Schmid factor (SF_LFmin) for slip system, where dislocations can glide long distances without interrupting the cementite plate. Additionally, the difficulty of slip propagation across colony boundaries was found to affect the deformability of the partially-spheroidized colonies. The spheroidized colonies exhibited the highest strain among the three types of colonies and were not correlated with the distance from the block boundary or SF_LFmin. Therefore, it was concluded that the degree of spheroidization was the main influence on the deformability of the colonies in the pearlitic steel. Meanwhile, the SF_LFmin, distance from the block boundary, and difficulty of slip propagation across the colony boundary sufficiently affected the deformability of the colonies, causing variation in the strain value for colonies with equal L_cem/L_col values.