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A unique bimodal microstructure is proposed to improve the mechanical properties of low‐carbon steels produced by twin‐roll casting. The ultrafine inclusions (0.2–2.0 μm) are produced in the as‐cast matrix. A high proportion of ultrafine acicular ferrite (UFAF) is induced due to these inclusions by controlling the critical processing factor, i.e., the cooling rate during γ → α transformation process. Therefore, heterogeneous microstructure is characterized by UFAF cluster, and network‐like coarse polygonal ferrite (PF) with a bimodal grain size distribution is engineered, significantly increasing the strength while not obviously damaging the plasticity. In addition, a new acicular ferrite (AF) heterogeneous nucleation mechanism is reported.

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