A new method for fast microstructure analysis of heavily deformed ferritic–pearlitic (F–P) steels has been developed based on X-ray diffraction (XRD). The method provides information about residual stresses and microstructure defects occurring within the material volume and therefore allows reliable estimation of strength properties. Besides, significant reduction of testing time can be achieved compared to conventional investigation methods. Development and testing of “fast microstructure analysis” was done by means of experimental simulation of wire and rod production process including hot rolling, wire drawing, and heat treatment of steel C45 with ferrite/pearlite microstructure. It was found that the dominant structure defects driving the ultimate tensile strength of F–P steel in case of hot-rolled and heated steel are the misfit dislocations in pearlite, while in the cold-drawn wires, it is the residual stress borne by the cementite lamellas.