The study investigates the significant back stress strengthening and strain hardening in gradient structured (GS) interstitial-free (IF) steel. Back stress, a long-range stress caused by the pileup of geometrically necessary dislocations (GNDs), is found to enhance both yield strength and strain hardening, thereby increasing ductility. The authors develop a simple equation and procedure to calculate back stress from the tensile unloading–reloading hysteresis loop, based on the physics of GND formation. The GS structure, characterized by a grain size gradient, induces a strain gradient that necessitates GND accommodation. The study uses a 1-mm thick IF steel sheet with a homogeneous coarse-grained (CG) microstructure annealed at 1173 K and subjected to surface mechanical attrition treatment (SMAT) to create a GS sample. Tensile tests reveal that GS samples exhibit higher ductility and yield strength compared to CG samples. The unloading–reloading hysteresis loops are analyzed to extract useful data for calculating back stress, and an equation is derived to accurately determine back stress values. The results show that GS samples have significantly higher back stress and strain hardening than CG samples, contributing to their superior mechanical properties.The study investigates the significant back stress strengthening and strain hardening in gradient structured (GS) interstitial-free (IF) steel. Back stress, a long-range stress caused by the pileup of geometrically necessary dislocations (GNDs), is found to enhance both yield strength and strain hardening, thereby increasing ductility. The authors develop a simple equation and procedure to calculate back stress from the tensile unloading–reloading hysteresis loop, based on the physics of GND formation. The GS structure, characterized by a grain size gradient, induces a strain gradient that necessitates GND accommodation. The study uses a 1-mm thick IF steel sheet with a homogeneous coarse-grained (CG) microstructure annealed at 1173 K and subjected to surface mechanical attrition treatment (SMAT) to create a GS sample. Tensile tests reveal that GS samples exhibit higher ductility and yield strength compared to CG samples. The unloading–reloading hysteresis loops are analyzed to extract useful data for calculating back stress, and an equation is derived to accurately determine back stress values. The results show that GS samples have significantly higher back stress and strain hardening than CG samples, contributing to their superior mechanical properties.