Plasticity-limiting factors and mechanisms in strip-cast low-carbon steel

The sub-rapid solidification and single-pass rolling characteristics of strip casting and rolling endow the product with a distinct microstructure and mechanical properties, markedly different from those of conventional hot-rolled products. A systematic comparison of the microstructure, tensile properties, and plasticity-limiting factors was conducted between commercially produced strip-cast-rolled low-carbon steel and conventional hot-rolled low-carbon steel. The results show that the strip-cast-rolled steel exhibits a fully ferritic structure with a gradient distribution of fine grains (average 28 μm) in the surface layer and coarse grains (average 50 μm) in the center, yielding an overall average grain size of 47 μm. The length fraction of grain-boundary strip-like carbides reaches 62.7%. In contrast, the conventional hot-rolled steel presents a uniform fine ferrite with a small amount of pearlite, an average grain size of 15.6 μm, and a grain-boundary carbide length fraction of only 14.9%. Thermodynamic calculations reveal that the rapid post-rolling cooling in strip-caststeel suppresses pearlite transformation, promoting carbon segregation to grain boundaries and the formation of continuous carbides, whereas the slower cooling in conventional hot rolling allows sufficient precipitation of carbon as pearlite. The elongation values are 31.6% and 39.8% for the strip-cast-rolled and conventional steels, respectively. The inferior plasticity of the strip-cast-rolled steel is primarily attributed to two factors: the poor deformation compatibility of coarse grains, which induces non-uniform deformation and stress concentration even at low strains; and the mismatch in deformability between brittle grain-boundary carbides and the ferrite matrix, making these carbides preferential sites for crack initiation. During deformation, larger grains deform preferentially, concentrating stress at the carbide/ferrite interfaces, where microvoids readily form and propagate into intergranular secondary cracks, with preferential nucleation at grain boundaries of large grains. Owing to its lower carbide content and more uniform microstructure, the conventional hot-rolled steel does not exhibit such preferential cracking in large grains.