Abstract: The chemical analysis and physical testing were processed on two low-elongation samples which were found in the regular testing experiment. One eligible sample was chosen to compare the difference among them. The results showed that the massive non-metallic inclustions which existed in the continuous casting slab caused internal stress between the substrate and the inclustions. The stress enable the crack to expand rapidly in the tensile test. The trans-grain cracks and pinholes formed in the process and finally caused low elongation of the sample.