Optimizing the balance between strength and toughness in low-carbon steel cold-rolled coils is a core challenge in materials science. Its essence lies in overcoming the traditional constraint of "high strength accompanied by low toughness" through microstructural control and process innovation. The strength improvement of low-carbon steel cold-rolled coils mainly relies on dislocation multiplication and work hardening introduced by cold deformation. However, excessive cold rolling can lead to dislocation entanglement and the formation of high stress concentration zones, which weakens toughness. Therefore, multi-scale structural design and process synergy are needed to maintain toughness reserves while improving strength.
Microstructural control is the key path to achieving this balance. Multiphase microstructure design introduces a composite structure of hard phases such as martensite and bainite with soft ferrite phases, utilizing the load-bearing capacity of the hard phases and the deformation coordination ability of the soft phases to achieve a synergistic improvement in strength and toughness. For example, in martensitic/austenitic dual-phase steel, martensite provides high strength, while austenite absorbs energy through transformation-induced plasticity (TRIP effect), delaying crack propagation. Furthermore, gradient structure materials, through a layered design of high strength on the surface and high toughness in the core, require cracks to overcome different performance layers during propagation, extending the energy dissipation path and significantly improving fracture toughness.
Grain refinement is another important method. According to the Hall-Petch relationship, reducing grain size can simultaneously improve strength and toughness. In fine-grained materials, crack propagation must cross more grain boundaries, and the tortuous path leads to increased energy consumption; simultaneously, grain boundaries, as barriers to dislocation movement, can prevent the concentration of plastic deformation, improving uniform deformation capability. For example, by combining controlled rolling and controlled cooling (TMCP) with microalloying (such as adding elements like Nb, V, and Ti), ultrafine ferrite grains can be formed in low-carbon steel, improving strength while maintaining good toughness.
Precise control of heat treatment processes is crucial for performance balance. Quenching, through rapid cooling to form martensite, significantly improves strength, but requires tempering to eliminate internal stress and avoid brittle fracture. Optimizing tempering temperature and time can control the morphology and distribution of carbide precipitation, improving toughness with minimal strength loss. For example, low-temperature tempering can retain some martensitic hardness while precipitating nanoscale carbides to hinder crack propagation; high-temperature tempering reduces hardness through carbide coarsening, but this requires a trade-off in strength loss.
The synergistic innovation of cold rolling and annealing processes provides new ideas for performance optimization. A research team in China proposed a "low-strain cold rolling + rapid low-temperature annealing" process, which, by precisely controlling 30% cold rolling deformation and rapid annealing at 550-670℃, successfully constructed a multimodal heterogeneous structure containing recovery grains, deformed grains, and recrystallized grains. The high dislocation density of the deformed grains provides back stress strengthening, while the soft phase characteristics of the recrystallized grains promote plastic deformation. The synergistic effect of these two processes allows the material to maintain a yield strength of 527 MPa while achieving an elongation of 14.2%, with a strength-plasticity product far exceeding that of conventional processes.
Microscopic defect control is also an indispensable aspect. Gases (such as hydrogen and nitrogen) and inclusions (such as oxides and sulfides) in steel significantly reduce toughness. Hydrogen-induced white spots and hydrogen embrittlement can be mitigated by reducing hydrogen content through vacuum smelting and ladle refining techniques; nitrogen hazards can be addressed by adding elements such as Al and Ti to form nitrides for fixation; oxide inclusions can be treated with Ca or modified with rare earth elements to achieve a spherical distribution, reducing stress concentration; sulfide inclusions can be alloyed with Mn to form ductile MnS, preventing lateral toughness degradation during hot rolling.
Modern materials science is driving the synergistic improvement of strength and toughness in low-carbon steel cold-rolled coils through cross-scale structural innovation and intelligent design. The integrated application of technologies such as nanoprecipitation strengthening, interface engineering, and phase transformation toughening enables materials to maintain stable performance even under extreme conditions. For example, deep-sea pipeline steel, through ultra-fast cooling processes to obtain an ultrafine bainitic microstructure, still maintains an impact energy of over 200J at -60℃, verifying the synergistic relationship between strength and toughness. In the future, with the improvement of materials genome databases and artificial intelligence prediction models, the performance optimization of low-carbon steel cold-rolled coils will become more precise and efficient, providing key material support for fields such as automotive lightweighting and energy equipment manufacturing.
