Relation between Low Elastic Limit and Mobile Dislocation Density in Ultra-low Carbon Martensitic Steel

Stress relaxation tests were conducted in the elastic region of an ultralow carbon martensitic steel (Fe–18%Ni alloy) to quantitatively analyze the effect of mobile dislocations on the low elastic limit of the steel. The elastic limit of the as-quenched material was measured at 255 MPa, although its tensile strength was as high as 720 MPa. The stress relaxation tests, which were performed at 255 MPa, revealed a remarkable stress reduction due to the movement of the mobile dislocations present in the as-quenched material. The total dislocation density barely changed during the test, while the distribution parameter (M-value) decreased significantly, indicating that the mobile dislocations exhibited stable arrangements. The 5% cold rolling before the relaxation tests suppressed the relaxation and simultaneously increased the elastic limit to a maximum, 435 MPa. By estimating the mobile dislocation density by relating the stress reduction in the stress relaxation tests to the distance of the dislocation movement evaluated via transmission electron microscopy (TEM) observations, it was estimated that the mobile dislocation density of the 5%-cold-rolled material was lowered to ~1/10 of that of the as-quenched material.