This book presents groundbreaking findings on how grain refinement affects phase transformations and the mechanical properties of ultrafine-grained steels. Grain refinement can significantly improve the strength and toughness of metals. Although the effect of grain refinement on material strength has been extensively studied, its impact on solid-state phase transformation in metals is not yet well understood. This book provides a systematic introduction to how grain size affects the solid-phase transformation of steels. This includes the pearlite, martensite, bainite and tempering transformations. A critical discovery is revealed that opens up a new approach to designing and producing high-strength steels at low cost: when grain size falls below 4 μm, traditional phase transition patterns change dramatically. Rather than forming lamellar structures, the pearlite transition produces granular cementite in a ferrite matrix, and the martensite transition shifts from twin to dislocation structures. This breakthrough enables the development of high-strength steels (2500–2600 MPa) with good ductility (11–7% elongation) using minimal alloying elements, offering a cost-effective approach to producing super-strong steels. This work will be an invaluable resource for academics and students of materials science, physics and mechanics, as well as engineers, technicians and researchers working in metallurgy and materials development.