It can be seen from the figure that the specimen NG crack is located in the bending part of the terminal, near the center of the curved arc, close to the front of the terminal, and the bending part of the crevice is the most severe position in the bend. The crack has a certain width and depth, and the crack edges are visible layered gap distribution.

2. Surface analysis

The two edges of the crack have overlapping characteristics, and the granular tin plateding distribution is visible in the layered gap of the crack, which indicates that the cracks have existed before the tin plating. The silhouette of the edge of the crack coincides, indicating that the crack position is intact before bending, that is, the crack is caused by bending when punching. Spectroscopic analysis shows that the local surface Zn content of the coating is high.

3. Metamorphic analysis

It can be seen from the figure that no obvious layering and other defects have been found at the bending, cracks can be seen at the bending, the cracks are trapezoidal, and there is no cracking extension at the bottom of the trap. Before plating Sn, the cracks already existed. The other crevice can see the "W"-shaped sharp angle, and the "W" sharp angle can be seen with the overlap contour of the gap, indicating that after the sharp angle of the crack here is stretched, the original layered gap is open.

Figure 5 shows the gold phase diagram after the corrosion of the specimen. It can be seen from the figure that the matrix metal phase can be seen in the distribution of twin crystals, the grain is fine, and the grain uniformity is better. In the bending deformation area, it can be seen that the grain is elongated, and the sharp angle area is close to the outer surface tissue, the core area of the substrate core and the grain in the uncurved area is still in an equal axis, that is, no more obvious deformation occurs.

4. Ingredient Analysis & Hardness Analysis

Table 1 shows that the chemical composition of the material meets the requirements of the C2600 grade chemical composition in JIS H3100-2018, and the cross-section hardness distribution is more uniform.

5. Bending analysis

Take OK samples along the bending process, low observation, as shown in Figure 6. It can be seen from the figure that the tin-plated layer of the bending and tensile surface is cracked, and the brass substrate is exposed. In order to observe whether the substrate is cracked, a metallographic analysis of the specimen is required.

The metallographic analysis of the bend sample will be bent, as shown in Figure 6. It can be seen from the figure that the tensile surface matrix of the bending part does not see the crack, and the compression surface can be seen to extend the thickness of the belt along the belt, and it can be seen that the tensile surface has a good extension performance under a certain bending and curvature when bending the curvature.

6. Discussion and Summary

The study believes that in the plastic deformation stage, copper and copper alloy grains will be slipping and forming steps, grain orientation, size is different, the degree of distortion is different, will form different surface conditions, such as smooth, wrinkles and cracks. It can be seen from the outline of the edge of the crack, the crack can be partially coincident, that is, the crack belongs to the external force to be separated. The metallographic phase can be seen, compared with the significant plastic deformation of the grain on the surface of the trapezoidal crack subsurface, the grain of the tissue of the sharp angle of the trapezoidal crack near the outer surface of the tension is not significantly elongated, that is, no more obvious plastic deformation occurs. After the crack subsurface is opened, the crack does not extend further in the direction of thickness and the bending test can prove that the material has excellent anti-stretching plastic deformation ability. The bending structure of the NG specimen has a layering defect, which is located in the subsurface layer, and the layering is first broken during the bending process.

To sum up, it can be seen that the defect here is the deep pressure on the surface of the strip, because the material has excellent plasticity, the subsequent rolling deformation, the tape extends at the crushing site, forming a layered defect.

Comprehensive analysis can be seen, the material subsurface has a layered defect, is the main reason for the cracks in the bending when the punching.