Step 1: Steel Behavior Under Stress Explained.
For structural steel, necking, a form of localized deformation, initiates after yielding on the stress-strain curve. Plastic deformation occurs post-yield point, with necking preceding the attainment of ultimate strength.Step 2: Summary.
Necking represents a concentrated decrease in the material's cross-sectional area. This process starts after the yield point is reached, and before the ultimate strength. Final Answer: \[ \boxed{\text{after yield strength}} \]