1.What is the core principle?
How the "total deformation" (total reduction rate) is distributed across multiple passes directly affects the work hardening behavior, internal stress state, texture (grain orientation) evolution, and subsequent annealing effect of the steel during the rolling process.
2.What are the direct effects on mechanical properties?
Strength and Plasticity (Work Hardening):
Each rolling pass introduces dislocations, causing the steel to work harden (become harder and more brittle).
Multi-pass, high reduction ratio distribution: If the total reduction ratio is mainly achieved in the first few passes, the material will reach a high hardening state early on, making deformation difficult in subsequent passes, potentially leading to edge cracking, and resulting in a lower plasticity reserve in the final product.
Uniform pass distribution: A more uniform reduction ratio distribution allows for a smoother work hardening process, which is beneficial for controlling the sheet shape and achieving a better balance between strength and plasticity in the final product.
Single-pass limit: Limited by mill capacity (rolling force, torque) and material deformation resistance, it is impossible to increase the single-pass reduction ratio indefinitely.
Yield Plateau and Elongation:
Annealing after cold rolling (recrystallization annealing) is crucial. The pass schedule affects the stored energy before annealing.
A well-designed pass pattern (combined with appropriate intermediate annealing) can promote the formation of uniform and fine recrystallized grains after annealing, thereby eliminating or reducing the yield plateau and improving elongation and stamping formability (such as deep-drawing steels DC04 and DC06, which have extremely high requirements for pass pattern design).
3.What are the effects on microstructure and anisotropy?
Texture Control:
Cold rolling creates specific deformation textures (such as α-texture and γ-texture).
Subsequent annealing transforms these textures into recrystallization textures. The final favorable texture (strong γ-texture, i.e., {111} planes parallel to the sheet surface) significantly improves the plastic strain ratio of the sheet.
Pass specifications (especially the reduction rate of the last few passes) play a decisive role in developing this favorable texture. Generally, a higher reduction rate in the final pass favors the formation of strong {111} texture.
Plastic Strain Ratio and Work Hardening Index:
r-value: Measures the sheet's resistance to thinning. A high r-value is crucial for deep-drawing parts. The total cold rolling reduction rate and pass allocation are the main process parameters controlling the r-value. For example, producing high r-value deep-drawing steel typically requires a total reduction rate of over 70%, and optimized pass design.
n-value: Characterizes the material's ability to deform uniformly. It is also influenced by the work hardening history accumulated over the passes and the microstructure after annealing.
4.What impact does it have on surface quality and plate shape?
Surface quality: Inappropriate pass distribution (such as excessive reduction in a certain pass) may amplify raw material defects or introduce new surface defects (such as chatter marks and scratches).
Strip shape (flatness): Uniform pass deformation helps maintain stable roll gaps and rolling conditions, thus achieving a good strip shape. Drastic pass variations increase the difficulty of strip shape control.
5.What are the design principles for the number of passes in actual production?
Commercial Grade (CQ): Prioritizes production efficiency and cost, with relatively simple pass design focused on ensuring basic mechanical properties and sheet shape.
Stamping/Deep Drawing Grade (DQ, DDQ, EDDQ): Pass design is a core secret. Typically employs:
High total reduction rate (e.g., 80%-85%).
Possibly using a complex "two-roll, one-anneal" process (i.e., first cold rolling + intermediate annealing + second cold rolling + final annealing) to better control texture.
Finely optimized reduction rate for each pass, especially the final pass.
High-strength steel (e.g., HSLA): While achieving the required strength, a certain degree of formability must be considered; pass design must balance work hardening and plasticity.