1.What are the causes of cracking in the flange area?
Location: Flange portion at the edge of the part or below the blank holder.
Characteristics: Cracks typically appear circumferentially or radially.
Possible Causes:
Excessive Blank Holder Force: Too high a pressure on the blank holder causes the material to be "locked up" during flow, preventing proper material replenishment and leading to tearing.
Material Anisotropy (Ear Direction): Cold-rolled coils exhibit significant anisotropy (e.g., large differences in plasticity in the 45° or 90° direction), resulting in insufficient material flow in certain directions.
Insufficient Die Corner Radius: Too small a corner radius on the die or blank holder leads to a significant increase in material flow resistance.
Poor Lubrication: Localized rupture of the lubricating film causes adhesive wear, resulting in tensile stress concentration.
2.What are the possible causes of sidewall cracking?
Location: On the straight or sloping wall of the part, usually parallel to the stamping direction.
Characteristics: Cracks are mostly longitudinal, and in severe cases, they penetrate the entire sidewall.
Possible Causes:
Insufficient Material Plasticity (Low Elongation): This is the most critical cause. The elongation or work hardening index (n-value) of the cold-rolled coil is too low to withstand the tensile deformation at this location.
Microstructural Defects: Severe banded structures or coarse carbides exist internally. During deformation, the interface between the carbides and the matrix becomes a crack initiation point.
Excessive Thinning Rate: The thinning amount at this location exceeds the design limit (e.g., the thinning rate exceeds the section reduction rate corresponding to the material's ultimate tensile strength).
Aging Embrittlement: The cold-rolled coil has been stored for too long (or has experienced abnormal temperatures), resulting in strain aging and a decrease in plasticity.
3.What are some possible reasons for cracks at the bottom rounded corners?
Location: At the transition fillet between the bottom surface and sidewall of the part.
Characteristics: The crack is arc-shaped and usually occurs in the most critical tensile area.
Possible Causes:
Insufficient relative thickness: The ratio of material thickness (t) to workpiece diameter (d) is too small (t/d value is too small), resulting in poor resistance to instability at the fillet.
Insufficient punch fillet radius: The fillet radius (Rp) is smaller than the minimum allowable bending radius of the material, leading to stress concentration.
Biaxial tensile stress state: The material is simultaneously subjected to radial and tangential tensile stresses at this location. If the yield strength ratio of the material is too high, it will quickly reach the fracture limit.
4.What are the possible causes of cracks at the base of the cross-shaped reinforcement or the bulge?
Location: At the root or edge of a localized forming feature (such as a reinforcing rib, bulge, or letterform).
Characteristic: The crack extends along the corner or edge of the reinforcing rib.
Possible Causes:
Combined Bending and Tension: During fine blanking or forming, the material at this location experienced severe combined shear and tensile stress.
Shear Surface Microcracks: In fine blanking parts, improper blanking clearance or insufficient pressure from the V-gear may have caused micro-tears (excessively large tear bands) on the shear surface, which are then expanded during subsequent forming.
Low Material Hardening Index: The material cannot disperse stress through work hardening after severe localized deformation.
5.How to analyze the unique material factors of cold-rolled coils (for fine stamping)?
Insufficient Spheroidization Rate:
Phenomenon: Insufficient spheroidization annealing of cold-rolled coils results in the presence of lamellar pearlite.
Consequence: Under stamping stress, lamellar carbides act like blades, cutting through the matrix and easily causing sidewall or corner cracking.
Surface Defects (Decarburized/Carburized Layer):
Decarburization: Surface decarburization reduces surface strength, leading to microcracks (crazing).
Carburization/Oxide Scale Indentation: Forms hard spots, disrupting matrix continuity and becoming crack initiation points.
Thickness Tolerance (Negative Tolerance):
Phenomenon: Actual material thickness is too thin (negative tolerance).
Consequence: Leads to a relatively larger die clearance, unstable material flow, especially during the stretching process, reducing the load-bearing cross-sectional area and increasing stress, resulting in cracking.
Residual Stress:
Phenomenon: Significant internal stress is introduced during the rolling and leveling processes of cold-rolled coils.
Consequences: The surface may be flat before stamping, but after stamping, the stress release combined with the working stress can lead to delayed cracking or immediate cracking.