1.What are the reasons related to the material itself?
Insufficient plasticity (low elongation): This is the most common cause. If the material's elongation (A80) is too low, it means it doesn't have enough "ductility" to accommodate the drawing depth of the die, and forced stretching will cause cracking.
High yield strength ratio: The yield strength ratio is the ratio of yield strength to tensile strength. If this ratio is too high (e.g., greater than 0.8), it means the force at which the material begins to deform is very close to the force at which it eventually breaks. This causes the material to thin quickly upon stretching, failing to transfer stress to the surrounding area in time, resulting in rapid localized stress concentration and cracking.
High hardness (insufficient annealing): If you consulted about hardness grades beforehand, pay attention. If the material grade is 1/8 hard (grade 8) or harder, and you are making parts that require deep drawing (usually grade A or annealed), cracking is almost inevitable. The material is too hard and lacks sufficient plastic flow capacity.
Significant anisotropy: Cold-rolled coils sometimes exhibit different properties in the rolling direction (longitudinal) and perpendicular direction (transverse). If the anisotropy of a material is too large, cracking is likely to occur in the direction of weakest strength when stretching circular or complex-shaped parts.
2.What are the causes of internal/surface defects?
Inclusions: Non-metallic inclusions within the steel strip can disrupt the continuity of the matrix, creating stress concentration points that act like "time bombs," leading to cracking.
Scratches: If the leveled surface has deep scratches (whether from rolling or leveling and conveying), these scratches themselves become stress concentration points, causing tearing along the scratches during stretching.
3.What are the reasons for the mold and process parameters?
Insufficient or worn die corner radius (R-angle): An excessively small radius on the punch or die will cause a significant increase in resistance during material flow, resulting in forced thinning and cracking at bends. Wear and roughening of the corner radius will also scratch the material surface and increase resistance.
Excessive blank holder force (BHF): The blank holder is used to prevent material wrinkling. If the blank holder force is too high, it will hold the material firmly in place, preventing it from flowing smoothly into the die from the side. The material in the middle will be forcibly stretched and thinned until cracking.
Insufficient die clearance: If the clearance between the punch and die is less than the sheet thickness, it will severely "grind" and thin the material, causing it to break.
Excessive draw ratio: When designing parts, if the draw depth is too deep relative to the part diameter (i.e., the draw coefficient is too small), exceeding the material's inherent draw ratio limit, cracking will be unavoidable.
4.What are the reasons for lubrication and operation?
Poor lubrication: The stretching process requires lubricating oil to form an oil film to reduce friction. If there is insufficient oil, the wrong type of oil is used, or the lubricating oil cannot form an effective oil film, the coefficient of friction increases, making material flow difficult and leading to cracking.
Inaccurate sheet positioning: If the blank is misaligned on the die, more material will flow on one side and less on the other. The side with less material will thin and crack due to overstretching.
Sheet metal edge condition: If there are burrs on the blanking or shearing edges, these burrs can become the starting point for micro-cracks in the early stages of stretching, which can then expand into tears.
5.How to troubleshoot and resolve this issue?
Examine the fracture surface:
If the fracture surface is gray and fibrous with obvious thinning at the edges (necking), it usually indicates insufficient material strength or excessive stretching.
If the fracture surface is flat and shiny, or has a herringbone pattern, and no obvious thinning, it usually indicates internal inclusions, micro-cracks, or external damage to the material.
Test hardness/material: Confirm that the material grade and hardness level match the requirements of the stretched part.
Check lubrication: Check if the oil film on the part surface is uniform and if there is any roughening on the die.
Adjust the blank holder force: Appropriately reduce the blank holder force or increase lubrication on the sheet material.