Balancing strength and toughness in cold-rolled low-carbon rigid strip is a core challenge in optimizing material properties, and the key lies in achieving synergistic improvement through composition design, process control, and microstructure regulation. Low-carbon steel typically has a carbon content below 0.25%, a characteristic that provides a foundation for high plasticity after cold rolling, but strength enhancement relies on the synergistic effect of alloying elements and processing techniques. For example, the addition of manganese significantly improves the hardenability of steel, enhancing strength through a dual mechanism of solid solution strengthening and grain refinement. Simultaneously, the MnS inclusions formed by manganese improve toughness and prevent crack propagation. Furthermore, the addition of silicon can increase the elastic limit of steel, but its content must be strictly controlled to prevent toughness loss; it is usually used in combination with manganese to balance performance.
Cold rolling process parameters have a decisive influence on microstructure evolution. During cold rolling, the strip undergoes severe plastic deformation, grain elongation, and the formation of high-density dislocations, leading to work hardening. At this point, the material strength is significantly improved, but toughness decreases due to dislocation pile-up and grain boundary weakening. To restore toughness, recrystallization is achieved through annealing. Precise control of annealing temperature and time is crucial: Too low a temperature leads to incomplete recrystallization and limited improvement in toughness; too high a temperature may cause grain coarsening, thus reducing toughness. For example, low-temperature continuous annealing can eliminate residual stress while maintaining a fine-grained structure, achieving a good balance between strength and toughness. Furthermore, the cooling rate after annealing needs optimization; rapid cooling can suppress the precipitation of second phases and avoid toughness loss.
Microstructure control is the core means of balancing strength and toughness. The typical microstructure of cold-rolled low-carbon rigid strips consists of a ferrite matrix with a small amount of cementite, and its properties depend on the ferrite grain size and cementite morphology. Refining ferrite grains can simultaneously improve strength and toughness: grain boundaries, as barriers to dislocation movement, can effectively hinder crack propagation, while a fine-grained structure further disperses stress by increasing the grain boundary area, delaying fracture. In addition, the morphology control of cementite is also essential. Spherical cementite causes less damage to toughness than lamellar cementite because its stress concentration effect is weaker. Controlled rolling and controlled cooling processes allow for dynamic adjustment of cementite morphology during rolling, achieving synergistic optimization of strength and toughness.
Deformation heat treatment is an effective way to improve overall performance. This process combines plastic deformation with heat treatment, introducing high-density dislocations and refining grains to achieve a dual improvement in strength and toughness. For example, short-time annealing after cold rolling promotes partial recrystallization, forming a mixed microstructure of fine equiaxed grains and deformed grains. This heterogeneous structure, through the synergistic deformation of hard and soft regions, ensures both strength and improved toughness. Furthermore, deformation heat treatment can induce martensitic transformation, generating dislocation-type lath martensite. Its high-density dislocations and fine substructure significantly improve strength, while the volume expansion generated by the martensitic transformation absorbs strain energy, inhibiting crack propagation and improving toughness.
Surface strengthening technology provides an additional path for improving toughness. Processes such as shot peening and rolling introduce residual compressive stress on the material surface, effectively inhibiting crack initiation and propagation. For example, shot peening can form a compressive stress layer hundreds of micrometers deep on the surface, with stress values reaching hundreds of megapascals, significantly improving fatigue life. Simultaneously, surface strengthening can refine the surface grains, forming nanocrystalline or ultrafine grain structures, further enhancing surface hardness and wear resistance. However, care must be taken to avoid over-strengthening leading to surface embrittlement; annealing is usually combined with this process to optimize the compressive stress distribution.
Precise control of chemical composition is fundamental to performance balance. In addition to carbon, manganese, and silicon, the addition of microalloying elements such as niobium, titanium, and vanadium can significantly improve strength and toughness. These elements refine austenite grains by forming carbonitride precipitates and nanoscale precipitates in ferrite, achieving precipitation strengthening. For example, the addition of niobium can refine the ferrite grain size to several micrometers, while the precipitates hinder dislocation movement through the Orowan mechanism, improving strength. Furthermore, strict control of harmful elements such as sulfur and phosphorus can reduce the number of inclusions, preventing crack initiation and thus improving toughness.
Achieving a balance between strength and toughness in cold-rolled low-carbon rigid strips requires the synergistic effect of composition design, process optimization, and microstructure control. By rationally adding alloying elements, precisely controlling cold rolling and annealing parameters, and employing deformation heat treatment and surface strengthening techniques, fine-grained, heterogeneous, and multiphase microstructures can be constructed. This significantly improves toughness while maintaining high strength, meeting the stringent requirements for comprehensive material performance in the automotive, construction, and other fields.
