In the present study, grain refinement of Fe–Si–Mn–Nb–B steel was achieved by high reduction hot rolling. Through the following ultra-fast cooling (UFC) and relaxation process, martensite-ferrite complexes with soft phase embedded in the hard phase were prepared. Extremely tiny ferrite grains significantly increased the toughness of the test steel, a large number of big-angle grain boundaries existing in fine martensite lath bundles effectively improved the strength and the strain hardening ability of the investigated material. The test steel with a relaxation time of 6 s shows the best comprehensive performance with a tensile strength of 1473 MPa, an elongation of 15.0%, a low yield ratio of 0.72 and a product of strength and elongation (PSE) of 22.10 GPa%.
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