1.How to control the temperature curve?
Temperature Gradient in the Heating Section
A slow heating pattern should be used to avoid excessive temperature differences between the inside and outside of the strip caused by rapid heating, which could generate thermal stress and lead to warping or deformation. The heating rate is typically controlled at 5-10°C/s, and the maximum heating temperature is set according to the steel grade (generally between 750-850°C) to ensure complete austenitization and uniform grain size.
Excessive temperatures may lead to coarsening of the grains and reduce the material's plasticity; insufficient temperatures prevent the elimination of cold rolling deformation stresses, affecting formability.
Soaking Section Holding Time
Sufficient time is required during the soaking stage (usually 30-60 seconds) to achieve uniform temperature across the strip. This allows interstitial atoms like carbon and nitrogen to fully form carbonitrides (such as TiC and NbC) with elements like titanium and niobium, achieving an "ultra-low carbon" effect (a core characteristic of IF steels), thereby reducing yield strength and improving ductility.
Excessively short holding times can result in incomplete compound precipitation, and residual interstitial atoms can increase the material's aging resistance (increasing strength and decreasing plasticity during subsequent use). Cooling Rate Control
During the cooling stage, rapid and uniform cooling (such as air jet cooling or water mist cooling) is required. The cooling rate is typically controlled at 20-50°C/s. Avoid prolonged dwelling in the 600-700°C range (where brittle structures such as pearlite are likely to form) to ensure the formation of a uniform ferrite structure.
Uneven cooling can lead to differences in properties between the upper and lower surfaces of the strip, or between the edges and center, affecting the consistency of subsequent stamping.
2.How to regulate tension control and board shape stability?
Tension Parameter Matching
During the continuous annealing process, strip tension must be strictly matched to thickness, temperature, and speed:
Inlet tension should not be excessive, otherwise it may cause strip deformation (especially since DC07 is soft and prone to plastic deformation).
In-furnace tension must be maintained stable to prevent strip deviation, wrinkling, or buckling due to tension fluctuations (DC07 has good ductility and is more sensitive to tension changes).
Tension ranges are typically set based on strip thickness (e.g., for 0.1-0.5mm thick strip, tension is controlled between 5-15kN).
Roller Accuracy and Stability
Guide rollers and guide rollers in the annealing furnace must maintain extremely high surface accuracy (Ra ≤ 0.8μm) and coaxiality to avoid surface defects such as indentations and streaks caused by roller scratches or runout (DC07 has stringent surface quality requirements, and minor defects can affect the appearance of subsequent coating or stamping).
Roller temperature must be monitored in real time to prevent thermal deformation from causing uneven roll surfaces, which could further affect strip shape.
3.How to regulate atmosphere control and surface quality?
Shielding Gas Composition
The annealing furnace must be filled with pure hydrogen or a hydrogen-nitrogen mixture (typically with a hydrogen content ≥5%) as a shielding atmosphere to prevent oxidation of the strip surface. The oxygen content must be strictly controlled below 50 ppm, and the dew point ≤ -40°C (to prevent moisture from causing surface oxidation, rust, or scale).
The shielding gas flow and pressure must be stable to prevent air from infiltrating the furnace (especially around the furnace mouth seal).
Surface Oil and Impurity Removal
Before annealing, rolling oil, iron powder, and other impurities on the strip surface must be thoroughly removed through electrolytic cleaning and alkaline cleaning.
Residual oil will carbonize during annealing, forming black spots or oil marks, affecting surface quality.
Residual iron powder will bond to the substrate at high temperatures, forming hard spots, reducing die life and product precision during stamping.
4.How to control speed and rhythm?
The continuous annealing line speed (typically 100-300 m/min) must be aligned with the heating, soaking, and cooling times to ensure the strip's residence time at each stage meets process requirements. Excessively high speeds can lead to inadequate heating and soaking, while excessively slow speeds can lead to coarse grains due to prolonged high temperatures.
Speed increases and decreases must be smooth to avoid tension fluctuations or temperature disturbances caused by sudden speed changes.
5.How to adjust the subsequent leveling process?
Annealed DC07 typically undergoes temper rolling (to an elongation of 1-3%) to eliminate the yield plateau (avoiding the formation of "Lüders lines" during stamping) and further improve the flatness. The rolling force and tension during the tempering process must be matched to the annealed material properties. Improper parameters can lead to hardening or surface scratching.