A cut edge has four distinct zones with different characteristics:
- Rollover – The plastically deformed zone bent as the cutting tools contact the edge of the sheet surface.
- Burnish – The zone where the cutting tools penetrate into the sheet metal, prior to any fracturing. The sheet metal stresses are such that the surface compresses into the cutting tool, which gives it a flat, smooth, and shiny appearance. The shear zone is another name for this region.
- Fracture – The zone where the cutting steels fracture the sheet metal, leading to separation from the remainder of the sheet. The surface of this region is rougher than the burnish zone, and is at an angle from the cutting direction.
- Burr – The metal elongated and pushed out on the trailing edge of the cut.
The cutting process also deforms the metal near the shear face, creating what is known as the shear affected zone (SAZ). The degree and influence of this deformation is a function of the cutting process (shearing, water jet, laser, and so on) as well as the steel microstructure and strength. Many studies conclude plastic deformation within the SAZ is a key contributor to sheared edge stretching failures.
The Shear Affected Zone is the area of work-hardened steel and microstructural damage behind the sheared edge. The deformation pattern in the SAZ includes a large shear-induced rotation of the grains that increases with proximity to the sheared edge. An etched cross-section such as that shown in Figure 1 highlights the grain rotation and gives a visual indication of the size of the zone.
Figure 1: Shear Face Components and Shear Affected Zone.K-1
Work hardening in the SAZ leads to a way to quantify the depth of the SAZ: by creating a hardness profile with readings starting at the edge and progressing deeper into the metal. The end of the SAZ occurs when the hardness readings level off to the bulk hardness.
Figure 2 compares the edge and hardness readings for CP800 and DP780, showing the depth of the SAZ for DP780 is about 41% of the initial sheet thickness of 1.56 mm, and the depth of the CP800 SAZ is 20% of the initial 2.90 mm sheet thickness.P-12
Figure 2: A) Sheared edge and SAZ of CP800; B) Sheared edge and SAZ of DP780; C) CP800 microstructure away from edge; D) DP780 microstructure away from sheared edge; E) hardness profile for the two sheared edges in the rolling direction.P-12
Plastic deformation from cutting and subsequent edge expansion forms micro-voids and creates other microstructural damage. The voids grow and combine with neighboring voids to create micro-cracks, which in turn combine with other micro-cracks resulting in the cracks that cause fractures in stampings.
Void nucleation in DP steels occurs through two mechanisms: decohesion of the ferrite-martensite interface or fracture of martensite islands.P-12, A-27, A-28 Figure 3 shows an example of both.
Figure 3: Aligned voids along loading direction. White solid square shows interface damage between martensite and ferrite. White dashed circles show voids formed by cracking of martensite.A-28
The study in Citation P-12 compared CP800 and DP780. The CP800 microstructure contains ferrite, bainite, and martensite. The DP780 microstructure has more martensite, with a larger strength differential between the phases, which combines to result in a lower nucleation strain and accelerated void nucleation compared with CP800. The DP grade has a larger SAZ, further promoting nucleation, growth, and coalescence of voids. This results in failure at a lower strain and leads to the lower edge stretchability of the DP780 compared to the CP800 alloy.P-12 The ductility of bainite restrains void initiation at high strains, which may play a role in improved sheared edge performance.S-24, S-25
Fracture initiation energy, a measure of fracture toughness correlates with hole expansion and stretch-flangeability as shown in Figure 4.Y-5
Figure 4: Correlation between fracture initiation energy and hole expansion ratio of various metals. Steels A – D are AHSS grades with Tensile strength ranging from 725 – 1000 MPa.Y-5
In addition to microstructural damage and fracture mechanics, simulation models improve in accuracy when the incorporating the effect of the temperature increase in the localized deformation zone. Simulation of the blanking of AISI 1050 shows a temperature at the cut edge of 440 °C. The edge temperature may be even higher in AHSS grades due to their higher strength.A-29
Figure 5: 1.27mm AISI 1050 steel blanked with 15% clearance. The simulation shows temperature, with the cut face getting as hot as 440 °C.A-29
When factoring in the considerations described here, simulation accuracy of hole expansion and sheared edge stretching improves significantly. Citations L-13 and L-14 provide additional background information about the Shear Affected Zone.