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process. Here, we present the surprising discovery that Fe-C martensite can also be formed inside a pearlitic steel, i.e., a ferrite-cementite composite without any austenite, by a new and unexpected route: severe mechanical deformation. [ 10 ] Current developments for improving the mechanical properties of metals, including steels, aim at producing nanostructured materials by severe plastic deformation (SPD). [ 11,12 ] To this end, cold-drawn pearlitic-steel wires have been very successful, reaching an ultrahigh tensile strength of up to ca. 7 GPa, [ 13 ] making them one of the world's strongest bulk materials. Pearlitic steels are used, for example, in large constructions such as suspension bridges. The high strength is associated with the refi nement of the originally lamellar eutectoid body-centeredcubic (bcc) α-Fe (ferrite) + Fe 3 C (cementite) structure of the pearlite, which leads to a nanocomposite that is stabilized by carbon segregation to the α-Fe grain boundaries. [ 13,14 ] With ongoing structure refi nement, more and more carbon must be accommodated inside the ferrite due to the dissolution of the cementite. [ 15,16 ] As a consequence, the concentration of carbon in the α-Fe exceeds the equilibrium solubility limit by far. It is generally assumed that a high density of vacancies and dislocations [ 17 ] accommodate the excess carbon. [18][19][20][21] A recent study by Taniyama et al., [ 22 ] revealed a tetragonal distortion of the ferrite lattice in heavily drawn pearlitic steel, which could be a consequence of the carbon supersaturation of the Fe matrix. Despite many studies on this topic, the accommodation of carbon in ferrite and a possible phase transformation are still controversial.In this study, we have combined atom-probe tomography (APT) and synchrotron X-ray diffraction (XRD) to study the carbon supersaturation of ferrite for two pearlitic steel-wire compositions -eutectoid and hypereutectoid. Knowledge of the carbon accommodation in the ferrite provides control to design the strength and ductility of nanostructured pearlitic steels. True drawing strains, ε , of 0 to 6.52 were analyzed, exceeding by far the drawing strains studied previously. [ 22 ] The two compositions, the high strains, the combination of advanced chemical and structural characterization methods, and a supporting ab-initio-based theoretical description show that a new mechanism of martensite formation is triggered under the extreme deformation conditions that occur in the SPD-induced structural refi nement of ultrahigh strength pearlitic steels. Deformation-driven nanoscale phase transformation provides a new way to tailor the mechanical properties of nanostructured steels and steel surfaces.The synchrotron XRD results of the cold-drawn pearliticsteel wires reveal signifi cant microstructural changes induced by SPD. We focus on the evolution of the diffraction peaks of ferrite with drawing strain rather than on cementite decomposition. The four major diffraction peaks, {110}, {200}, {211} and Steel is the dominant structu...