Deformation Behavior at Low Temperature in 9mass%ni Steel

The strain distribution in the 9 mass%Ni steel introduced by tensile deformation at cryogenic temperature was visualized using a digital image correlation method, and the relationship between the strain distribution and the microstructure of the steel was systematically investigated. Based on the obtained results, the factors that influence the strain distribution were discussed. In the present 9mass%Ni steel, regions consisting of tempered- and fresh-martensite and austenite (TFMA) were embedded in the tempered martensite matrix. The volume fraction of the retained austenite varied along the normal direction of the hot-rolled plates, indicating that Ni segregation occurred during the manufacturing process. As the tensile stress increased, the total elongation remained constant with decreasing temperature. Strain was introduced inhomogeneously via tensile deformation at 77 K. The high- and low-strain regions tended to be distributed in a unit of the block, indicating that the deformability differed among the blocks. The average strain in the block (eblock) was strongly correlated with the Schmid factor of the slip system on the habit plane (SFhabit) and area fraction of TFMA in a block (AMA). The least absolute shrinkage and selection operator regression revealed that the contributions of SFhabit and AMA to Eblock were nearly equal. (1) Therefore, the deformability of the block in the 9 mass%Ni steel is dominated by SFhabit and A(TFMA)