Engineering Materials and Their Applications
Coining of Pierced Hole with Aid of Scrap
2022 Volume 63 Issue 2 Pages 240-246
Details
2022 Volume 63 Issue 2 Pages 240-246
The tensile residual stress near pierced holes deteriorates the fatigue properties and hydrogen embrittlement resistance of automotive parts. Consequently, a simple shearing method that can reduce this type of stress is highly desired in the automotive industry. In this study, the effect of a coining method using scrap material on the stress near a 10 mm diameter pierced hole was investigated. The fracture surfaces of the pierced scrap material and pierced hole presented similar shapes because these parts were produced by the separation of a single sheet via crack propagation. Therefore, it is possible to use the pierced scrap material as a precise coining tool. The effectiveness of this method was investigated for 980 MPa grade steel sheets with a thickness of 1.6 mm. To examine the effect of the coining stroke, the test was conducted at two levels: a coining stroke in which the product and scrap were at the same level and a coining stroke in which the scrap passed through a hole in the product. The tensile residual stress on the sheared surface significantly reduced as the length of the coining stroke increased. The results of finite element method simulations indicated that the above effects were mainly caused by the change in the shape of the pierced hole and the stress generated by the contact between the pierced scrap material and the hole in the product.
This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 301–306. The Abstract is slightly modified.
Fig. 1 Schematic of proposed method.
In recent years, the automotive industry has actively promoted the application of high-strength steel sheets, mainly for vehicle bodies, to reduce the environmental impact and improve the crash-worthiness of vehicles. In general, high-strength steel sheets exhibit a higher strength and can absorb more energy during crash deformation compared to those of traditional steel sheets of the same thickness. Therefore, 1.5 GPa hot-stamping materials and 1180 MPa grade cold-rolled steel sheets have been developed and put into actual use.1) However, weight reduction studies using aluminum alloys and plastic materials with low specific densities are being actively conducted. Compared to those using these lightweight materials, part processing and vehicle body assembly using high-strength steel sheets do not require major changes in the equipment and manufacturing technology, and it is thought that manufacturing costs due to weight reduction is relatively small. However, in the case of press forming, both springback and wrinkles increase as the strength of the steel sheet increases, making it difficult to ensure the dimensional accuracy of parts.2) In addition, as the strength of the steel sheet increases, the uniform elongation and fracture limit decrease, whereas the notch sensitivity increases. Thus, the risk of fracture due to stretch-flange deformation of the sheared edge increases, and the fatigue strength and hydrogen embrittlement resistance of pierced holes decrease.3–5) Consequently, it is typically challenging to achieve both high vehicle body performance and high productivity using high-strength steel sheets, and these technical production problems must be solved to expand the application of high-strength steel sheets. Numerous methods have been studied to improve the hole-expansion ratio and fatigue strength of pierced holes.6–12) These studies can be broadly classified into those that improve the properties by changing the shape of the punch and those that improve the properties through post-processing, such as shaving and coining. Among these, the coining process is a method that improves various properties by processing a pierced hole using a spherical or conical punch. Several studies have been conducted on the effect of the coining process on the residual stress of a pierced hole. For example, Nakagawa et al.6) and Abe et al.7) clarified that coining the pierced part smooths the sheared edge and improves the hole-expansion ratio. In addition, Shirasawa et al.8) reported that the coining process reduces the tensile residual stress in the pierced hole and improves the fatigue properties.
Furthermore, Tamura et al.9) used the finite element method (FEM) to analyze the change in the residual stress in a pierced hole during coining and investigated the mechanism.
Because the degree of improvement achieved using these coining methods depends on the punch shape, it is necessary to select an appropriate punch shape for the shape of the sheared surface. However, the blanking of automobile parts often involves various piercing clearances and cutting line shapes, which affect the shape of the sheared edge. Therefore, it is difficult to obtain consistent properties using coining punches having the same shape.7) Furthermore, to reduce the variation in the coining effect on a pierced hole, it is essential to match the positions of the tool and cutting line with high accuracy.
Therefore, in this study, we proposed a new coining process that utilizes the scrap generated during piercing as a coining punch to solve the abovementioned production engineering problems. The effect of the proposed coining process was clarified by a simple experiment. Furthermore, the mechanism by which the residual stress decreased was analyzed using the FEM.
Figure 1 illustrates the outline of the proposed method. In shearing (such as piercing and trimming), the scraped part is generally discarded; however, in the proposed method, it is used as a tool. When a steel sheet is cut by shearing, the angles of the fracture surfaces of the scrap and product match, regardless of the shearing conditions. Therefore, after shearing, the steel sheet on the scrap side is used as a coining punch, and the fracture surface on the scrap side is pressed against the product side with a cushion pin or similar instrument. Consequently, the tensile residual stress on the sheared edge is reduced by the coining effect. In this study, piercing and coining tests were conducted separately to investigate the effect of the proposed method using a simple test, and the relationship between the coining stroke and residual stress of the pierced edge was examined.
Schematic of proposed method.
The test material was a 980 MPa grade cold-rolled steel sheet with a thickness of 1.6 mm, and its mechanical properties are listed in Table 1. The mechanical properties were evaluated by conducting a tensile test using an Instron-type testing machine at a crosshead speed of 3 mm/min (strain rate of 1 × 10−3 s−1). For the tensile test, a JIS Z2241 No. 5 test specimen was prepared in a direction transverse to the rolling direction.
Using the tool shown in Fig. 2, a hole with a diameter of 10 mm was pierced in the test material, and the steel sheet was separated into the product and scrap. Here, the clearance between the punch and die was 0.185 mm (sheet thickness ratio of 11.6%), and a servo press was used to lower the punch to penetrate the die hole by 1 mm at a speed of 100 mm·s−1.
Piercing experiment.
After piercing, the scrap and product were placed as shown in Fig. 3, a load was applied to the scrap using the same tool as that in the piercing test, and the pierced hole was coined. To investigate the relationship between the coining stroke and the residual stress of the sheared edge, the scrap was pushed into the pierced part and subsequently removed. Two values were set for the coining stroke: S1 = 1.6 mm, which is equal to the thickness of the product, and S2 = 3.2 mm, in which the scrap penetrates the pierced hole.
Coining experiment.
A cross-section of the pierced surface was cut, on which Vickers hardness measurements were conducted. The positions of the hardness measurements are shown in Fig. 4. The indentation load was 0.98 N, and the measurements were conducted at 0.1 mm intervals at a distance of 0.08 mm from the pierced surface.
Position of hardness measurements on the pierced edge.
The residual stress in the pierced hole was measured using X-rays. Because it is difficult to irradiate X-rays on the surface of a circular hole, the pierced hole was cut into a semicircle, and the residual stresses in the sheet thickness and circumferential directions were measured at the bottom of the semicircle. The measurement in the cross-section was taken at the center of the sheet thickness, the spot diameter was 0.5 mm, and the measurement was conducted using the parallel tilt method. Specifically, the change in the diffraction angle of the diffraction line with the angle, Ψ, formed by the sample and lattice surface directions was measured, and the residual stress was calculated from the gradient of the 2θ–sin2 Ψ plot.13)
Figure 5 shows surface and cross-sectional photographs of the pierced hole before and after coining. Figure 5(a) shows the sheared surface after piercing. The sheared edge consisted of 4.6% droop, 21.7% sheared surface, and 73.7% fracture surface relative to the sheet thickness. Figure 5(b) shows photographs of the pierced hole after coining by stroke S1, in which the scrap and product were at the same height. The boundary between the sheared and fracture surfaces became ill-defined, and sliding marks were observed on the fracture surface between the scrap and product. Figure 5(c) shows photographs of the pierced hole after coining with stroke S2, in which the scrap penetrated the pierced hole. The pierced part was significantly deformed, the droop disappeared, and a burr appeared on the droop side.
Sheared surface before and after coining (S1: Coining stroke at which the product and scrap were on the same level; S2: Coining stroke at which a scrap passed through the hole of the product).
Table 2 lists the details of the residual stress measurements. At the as-pierced edge, high residual stresses were generated in both the circumferential (σc) and sheet thickness (σt) directions, which were close to the tensile strength of the steel sheet. In contrast, on the sheared surface coined by stroke S1, the tensile residual stress decreased substantially in both the circumferential and sheet thickness directions. On the sheared surface coined by stroke S2, the tensile residual stress decreased further, and compressive residual stresses were measured in both the circumferential and sheet thickness directions.
Figure 6 shows the distributions of the measured hardness of the pierced hole before and after the two coining processes. The hardness of the as-pierced sheared edge was larger on the fracture surface than that on the sheared surface. Compared with that of the as-pierced sheared edge, the hardness of the sheared edge coined by stroke S1 increased, and the magnitude of the increase in the central region of the sheet thickness was relatively large. Furthermore, when coining was performed by stroke S2, the hardness near the sheared surface increased considerably.
Surface hardness of pierced edges.
Table 3 lists the average Vickers hardness under the three conditions. Relative to the hardness of the as-pierced hole, coining by strokes S1 (in which the scrap and product were at the same height) and S2 (in which the scrap penetrated the pierced hole) increased the hardness by 4.2 Hv and 12.9 Hv, respectively.
The mechanism by which piercing with scrap reduces the tensile residual stress at the pierced hole is discussed in this section. There are two possible causes for the reduction in residual stress. One is that coining the hole-pierced part using the scrap causes the hole to expand, and the subsequent elastic recovery causes the hole to shrink when the scrap is removed. The other is that the contact between the scrap and the product generates local compressive stresses in the hole-pierced part of the product. To consider these factors, the changes in the residual stress were analyzed using the FEM.
5.1 Analysis model and conditionsCoining with scrap was numerically analyzed by conducting simulations using the static implicit method FE code, ABAQUS/Standard.14) To quantitatively evaluate the coining effect after piercing, it is necessary to conduct a piercing simulation based on the ductile fracture theory considering damage as well as a coupled analysis of the subsequent coining. However, because the crack growth and residual stress during piercing cannot be predicted accurately, only the coining after shearing was analyzed.
First, a two-dimensional axisymmetric model was built from the cross-sectional image of the part after piercing (Fig. 5(a)). Outlines were extracted at intervals of approximately 0.3 mm, and the experimental shapes of both the pierced hole and scrap after piercing were introduced. Figure 7 depicts an outline of the analysis model. In the model, the tool was considered as a rigid body and the steel sheet was considered as an elastoplastic body. In addition, the axially symmetric four-node reduced integration element, CAX4R, was used for the steel sheet, with a minimum size of 0.01 mm. Coulomb friction was assumed between the tool and steel sheet and between the pierced hole and scrap, and the friction coefficient was set to 0.12. The work-hardening properties of the steel sheet were determined by approximating the equivalent stress–equivalent plastic strain curve obtained from the tensile tests conducted in a direction transverse to the rolling direction using the Swift hardening law as expressed in eq. (1).
| \begin{equation} \sigma_{\textit{eq}} = 1624(\varepsilon_{\textit{eq}} + 0.0045)^{0.149}, \end{equation} | (1) |
FEM analysis model (Element set a: Elements on fracture surface; Node set b: Nodes located at a radial distance of 1.5 mm from the sheared surface).
The equivalent plastic strain was estimated from the Vickers hardness measurements of the hole-pierced part (Table 3). Assuming that the equivalent stress is 3.3 times the Vickers hardness,16) the magnitude of the equivalent plastic strain was estimated from the Swift hardening law in eq. (1), which was used as the initial condition for element set a. The equivalent plastic strain was estimated to be 0.102 from the average hardness of the sheared surface after piercing (Table 3).
5.2 Changes in residual stress during coiningTo examine the change in stress during the coining process, the previously described test was numerically simulated. First, after the punch shown in Fig. 7 was displaced in the z-direction, the contact between the product and scrap was released, and the unloading process was analyzed. Similar to that in the experiments, there were two depths by which the scrap was pushed into the pierced hole: the scrap was pushed to the same height as that of the product (S1) and the scrap was pushed until it penetrated the pierced hole (S2).
Figure 8 depicts the relationship between the coining stroke and average displacement in the r-direction ($\overline{U_{r}}$) of the node set at the hole edge. When the scrap was pushed to the same height as that of the product (S1), the hole-edge displacement ($\overline{U_{r}}$) initially increased as the coining stroke (s) increased and subsequently decreased during the unloading process. In contrast, when the scrap was pushed until it penetrated the pierced hole (S2), the maximum hole-edge displacement ($\overline{U_{r}}$) occurred when the coining stroke (s) was 2 mm. When the scrap was pushed further, the hole-edge displacement ($\overline{U_{r}}$) decreased. During the subsequent unloading process, no significant change was observed in the hole-edge displacement ($\overline{U_{r}}$). When the scrap was pushed to the same height as that of the product (S1), the unloading process began as the hole expanded. Therefore, the hole was considered to contract under the elastic recovery during the unloading process, causing the hole-edge displacement ($\overline{U_{r}}$) to decrease. In contrast, when the scrap was pushed until it penetrated the pierced hole (S2), the hole shrank to some extent during the scrap penetration. Therefore, no significant change was observed in the hole-edge displacement ($\overline{U_{r}}$) during the unloading process.
Radial displacement of fracture surface during coining process (U: Initiation of unloading; S1: Coining stroke at which the product and scrap were on the same level; S2: Coining stroke at which a scrap passed through the hole of the product).
Figure 9 depicts the relationship between the hole-edge displacement ($\overline{U_{r}}$) and circumferential stress. The circumferential stress was evaluated using the average circumferential stress ($\overline{\sigma _{c}}$) of a single surface element layer (element set a) of the pierced hole. When the scrap was pushed to the same height as that of the product (S1), the circumferential stress ($\overline{\sigma _{c}}$) decreased as the hole-edge displacement ($\overline{U_{r}}$) increased. After unloading (when the hole-edge displacement ($\overline{U_{r}}$) decreased under the elastic recovery), the average circumferential stress ($\overline{\sigma _{c}}$) decreased to approximately zero. This corresponds to the shrinkage phenomenon due to the springback occurring when the scrap is removed from the expanded hole. In contrast, when the scrap was pushed until it penetrated the pierced hole (S2), the average circumferential stress ($\overline{\sigma _{c}}$) continued to decrease to values lower than those when the scrap was pushed to the same height as that of the product (S1). Although this result qualitatively reproduces the variation in the residual stress measured by X-ray diffraction, there is no quantitative agreement. In this numerical simulation, the initial stress and equivalent plastic strain based on the measured results were applied to a single element layer on the fracture surface. Therefore, the distributions of the plastic strain and stress in the hole were not considered, and the damage to the pierced edge was insufficiently accounted. Consequently, the experimental results were not predicted quantitatively.
Circumferential stress during coining process (U: Initiation of unloading; $\overline{\sigma _{c}}$: Average circumferential true stress of element set a; S1: Coining stroke at which the product and scrap were on the same level; S2: Coining stroke at which a scrap passed through the hole of the product).
It is presumed that the change in the average circumferential stress ($\overline{\sigma _{c}}$) is caused by the combined effect of the local compressive stress on the pierced hole by coining, expansion of the hole by the scrap, and shrinkage during unloading. Among these, the first is probably generated by the material in the surface layer of the hole-pierced part restraining the deformation in the circumferential direction during expansion in that direction because of the contact between the scrap and hole-pierced part.
To classify and evaluate these complex factors in the coining process using scrap, the effects of the expansion of the hole edge and shrinkage during unloading were verified by numerical analysis. To analyze the change in the average circumferential stress ($\overline{\sigma _{c}}$) during unloading, node set b (which was sufficiently distant from the pierced edge) was forcibly displaced in the r-direction and subsequently released. The magnitude of the forced displacement on node set b (Δrmax) was determined based on the results of the coining analysis shown in Fig. 8. When the scrap was pushed to the same height as that of the product (S1), Δrmax was 0.0209 mm, and when it was pushed until it penetrated the pierced hole (S2), Δrmax was 0.0241 mm.
Figure 10 depicts the relationship between the average displacement in the r-direction ($\overline{U_{r}}$) of node set b and the equivalent plastic strain. The equivalent plastic strain was evaluated using the average value ($\overline{\varepsilon _{\textit{eq}}}$) of a single surface element layer (element set a) of the pierced hole. Immediately after the start of the deformation, the equivalent plastic strain ($\overline{\varepsilon _{\textit{eq}}}$) remained constant with increasing displacement ($\overline{U_{r}}$); subsequently, it increased with increasing displacement ($\overline{U_{r}}$). During the unloading process, the equivalent plastic strain ($\overline{\varepsilon _{\textit{eq}}}$) remained constant as the displacement ($\overline{U_{r}}$) decreased. The constant equivalent plastic strains ($\overline{\varepsilon _{\textit{eq}}}$) both immediately after the start of the expansion of the hole and during unloading were due to elastic deformation.
Equivalent plastic strain during expansion and reduction of pierced hole (U: Initiation of unloading; $\overline{\varepsilon _{\textit{eq}}}$: Average equivalent plastic strain of element set a).
Figure 11 depicts the relationship between the average displacement in the r-direction ($\overline{U_{r}}$) of node set b and the stress in the circumferential direction. Similar to the previous analysis, the circumferential stress was evaluated using the average circumferential stress ($\overline{\sigma _{c}}$) of a single surface element layer (element set a) of the pierced hole. As the displacement ($\overline{U_{r}}$) of the node set increased (i.e., as the hole is expanded), the circumferential stress of the pierced hole increased. As the deformation progressed, the rate of increase in the circumferential stress ($\overline{\sigma _{c}}$) decreased (i.e., the increase became more gradual). Subsequently, the hole shrank, and the circumferential stress ($\overline{\sigma _{c}}$) decreased during the unloading process. Comparing the circumferential stresses ($\overline{\sigma _{c}}$) before and after deformation, the circumferential tensile stress after unloading was lower than that before deformation when the displacement boundary conditions in the r-direction were applied.
Circumferential stress during expansion and reduction of pierced hole (U: Initiation of unloading; $\overline{\sigma _{c}}$: Average circumferential true stress of element set a).
Based on the analysis results thus far, the change in the circumferential residual stress ($\overline{\sigma _{c}}$) is larger during the coining process than that when the hole is simply expanded and contracted. Focusing on the circumferential stress as the hole expands, the circumferential stress increases when the hole is expanded by applying forced displacement. In contrast, the circumferential stress decreases during the coining process because the compressive stress due to the contact between the scrap and product exceeds the circumferential tensile stress introduced by the expansion of the hole.
Thus, the reduction in circumferential tensile residual stress caused by coining using scrap can be interpreted as a combination of the elimination of tensile residual stress by shrinkage due to elastic recovery after the expansion of the hole and application of local compressive stress due to the contact between the scrap and product.
When the scrap penetrates the product, the residual stress is dependent on the sliding phenomenon between the scrap and pierced part, and it is presumed that the residual stress in the sheet thickness direction (which is the punch stroke direction) is most strongly affected by this. To consider this through numerical simulation, it is necessary to predict the sliding phenomenon between the scrap and pierced part with high accuracy after reconstructing the microscopic damage of the sheared edge. However, because there are numerous challenges in conducting this numerical analysis using the FEM, quantitative consideration of the change in residual stress in the sheet thickness direction is a topic for future research.
To reduce the tensile residual stress of pierced holes, we proposed a shearing method that utilizes the scrap generated by piercing as a coining punch. The effect of the proposed method was examined experimentally, and the mechanism of the reduction in the residual stress was analyzed by numerical simulation of the coining processing using scrap. Based on the above, the following findings were obtained:
