The finishing rolling elongationδof the alloying non-quenched and tempered steel is calculated with the covalent electron number nA of the strongest bond of the alloying phases and the interface electron density differenceΔρof the phase interfaces. The calculations show that the elongationδα-Fe of the matrixα-Fe decreases with rolling refinement, the elongationδα-Fe-C-M of solid solution phases (M denotes alloying element) is inversely proportional to the covalent electron number nAα-Fe-C-M of the strongest bond, the elonga tion decrementΔδα-Fe/α-Fe-C-M caused by interface strengthening is directly proportional to the interface electron density differenceΔρα-Fe/α-Fe-C-M, but the elongation decrementsΔδα-Fe/MCC1 andΔδα-Fe-C/MCC2 caused by dispersion strengthening and precipitation strength- ening respectively are directly proportional to the ratio of the electron density differenceΔρα-Fe/MCC1 andΔρα-Fe/MCC2 of the strengthening interfaces toΔρα-Fe/α-Fe-C of the basic interfaceα-Fe/α-Fe-C. Therefore, the finishing rolling elongation of the alloying non-quenched and tempered steel is considered to be subtracting all the elongation decrements of solution strengthening, interface strengthening, dispersion strengthening and precipitation strengthening from the elongation of the refinedα-Fe matrix. The calculation formulas in this paper are integrated with the proposed ones ofσS,σb and ofαK delivered in another paper, the finishing rolling mechanical properties can be achieved and the calculated results agree well with the measured ones. The finishing slip elongation δ of the alloying non-quenched and tempered steel is calculated with the covalent electron number nA of the strongest bond of the alloying phases and the interface electron density difference Δρ of the phase interfaces. The calculations show that the elongation δα-Fe of the matrix α- Fe decreases with rolling refinement, the elongation δα-Fe-CM of solid solution phases (M denotes alloying element) is inversely proportional to the covalent electron number nAα-Fe-CM of the strongest bond, the elongation decrement Δδα-Fe / α-Fe -CM caused by interface strengthening is directly proportional to the interface electron density difference Δρα-Fe / α-Fe-CM, but the elongation decrements Δδα-Fe / MCC1 and Δδα-Fe-C / MCC2 caused by dispersion strengthening and precipitation strength- ening respectively are directly proportional to the ratio of the electron density difference Δρα-Fe / MCC1 and Δρα-Fe / MCC2 of the strengthening interfaces to Δρα-Fe / α-Fe-C of the basic interface α-Fe / α-Fe-C. Thus, the finishing rolling elongation of the alloying non-quenched and tempered steel is considered to be subtracting all the elongation decrements of solution strengthening, interface strengthening, dispersion strengthening and precipitation strengthening from the elongation of the refined α-Fe matrix. The calculation formulas in this paper are integrated with the proposed ones of σS, σb and of α K delivered in another paper, the finishing rolling mechanical properties can be achieved and the calculated results agree well with the measured ones.