1.Where do cold-rolled coil stampings most commonly crack? Why?
The most common cracking sites are concentrated in the flange area, sidewalls (especially near the punch fillet), and the bottom fillet transition area. This is because:
Flange area: During deep drawing, it bears significant tangential compressive stress and radial tensile stress. If the material's plasticity is insufficient or the sheet thickness deviation is large, edge cracks are prone to occur.
Sidewall and punch fillet area: This is the area of most severe deformation. The material must withstand both bending and tension. If the material's elongation or local forming performance is poor, cracking is highly likely to occur here.
Bottom fillet: Here, the material contacts the punch, resulting in high friction and the most severe thinning. If the material strength is insufficient or lubrication is poor, bottom cracking is likely to occur.
2.Why do cracks often occur on the sidewalls of stamped parts, and why are the crack directions sometimes longitudinal?
Insufficient Bending: If the die fillet radius is too small, the material will experience severe bending and stretching as it flows through the fillet, leading to excessive tangential stress on the outer surface, exceeding the material's limits and causing cracking.
Material Anisotropy: During the rolling process, cold-rolled coils develop texture, resulting in different mechanical properties in different directions within the sheet plane. When the stamping direction does not match the rolling direction, the direction with the lower plastic strain ratio (r-value) has poor deformation capacity and is prone to longitudinal cracks on the sidewalls.
Performance Differences Between the Beginning and End of Cold-Rolled Coils: If the stamped part happens to be sourced from the beginning and end of the cold-rolled coil where performance fluctuates, its plasticity is poor, making it more prone to failure in the sidewall areas where deformation is concentrated.
3.Is the location of the crack directly related to the performance fluctuations of the cold-rolled coil itself (such as differences between the beginning and end)?
Inhomogeneous properties lead to strain concentration: During stamping, the high-strength, low-plasticity sections (usually the beginning and end) and the low-strength, high-plasticity middle section deform in uncoordinated ways. The poorly plastic sections cannot deform in tandem and will reach their limit first, becoming the starting point for cracking.
The influence of positioning point selection: If cracking occurs at a fixed location, the original position of the stamped part on the cold-rolled coil can be traced. If this location corresponds precisely to an abnormal section of the hot-rolled raw material's coiling temperature or a section of fluctuating cold-rolling tension, it can be determined that the inhomogeneous properties of the raw material led to localized deterioration of formability.
4.How can we infer from the location and shape of cracks whether the problem lies with the raw materials or the stamping process?
Crack Morphology:
Raw Material Issues: Cracks are typically irregular or serrated, with a relatively flat fracture surface, and often accompanied by insignificant necking (characteristics of brittle fracture). On multiple stamped parts from the same coil, the crack location may not be fixed, but it always appears at the corresponding head and tail sections with poorer performance.
Process Issues: Cracks are usually regular, opening along a fixed direction (e.g., along a 45° angle), with obvious necking at the fracture surface (ductile fracture). The crack location is very consistent, such as always being at the same rounded corner or the same sidewall location.
Location Distribution:
If, on the same steel coil, the cracking rate of the head stamped parts is much higher than that of the middle, or if the crack locations are concentrated at the beginning and end of the feeding direction, it can generally be determined that it is due to the performance difference between the head and tail of the cold-rolled coil.
If the crack location highly coincides with a certain wear point or uneven blank holder force area in the die, it is mostly a process or die issue.
5.What are the troubleshooting steps for analyzing the location of cracks in cold-rolled coil stampings?
**Location Sampling:** First, record the specific location of the cracked part in the stamping direction (e.g., distance from the billet edge, at which fillet radius). Then, trace the original position of the billet on the cold-rolled coil (head, middle, or tail).
**Performance Re-testing:** Take samples from near the cracked part and from normal areas far from the crack for mechanical property (yield strength, tensile strength, elongation) and metallographic analysis. If the elongation of the cracked area is significantly lower or the microstructure is abnormal (e.g., coarse and uneven grains), it indicates a problem with the raw material.
**Thickness Comparison:** Measure the thickness reduction rate of the cracked area. If the thinning rate far exceeds the material's allowable limit and is unevenly distributed, it may indicate excessive thinning due to poor local material properties.
**Fracture Analysis:** Observe the fracture surface using a scanning electron microscope to determine whether it is dimple fracture (ductile fracture, often due to manufacturing processes) or cleavage or intergranular fracture (brittle fracture, often due to material defects).
Process troubleshooting: Simultaneously check whether the mold clearance, blank holder force, and lubrication conditions are consistent. After eliminating process fluctuation factors, then identify the responsibility for the differences between the beginning and end of the raw materials.