Scattering Behavior of Slivers in Shearing of Magnetized Ultra-High-Strength Steel Sheets

Abstract

The changes in the magnetization properties of high-strength steel and ultra-high-strength steel sheets are investigated, and then the sheared edges and the scattering behavior of slivers in shearing of the ultra-high-strength steel sheets are observed. The maximum magnetic flux density of the magnetized sheet is increased with the increasing tensile strength of the sheet. The maximum magnetic flux density in the magnetized blanks decreases, whereas the density in the demagnetized blanks increases. In the sheared edges, the ratio of the fracture surface becomes larger with the increasing tensile strength of the steel sheet. In shearing, the shearing slivers are observed at the time of crack penetration and at the time of punch rise. The mass of the slivers generated from the blank in shearing increases with the increasing tensile strength of the steel sheet. Two-thirds of the generated shearing slivers stick to the blank in the magnetized blank, whereas two-thirds of the slivers in the blank without magnetization scatter to the outside of the die.

1. Introduction

To improve the fuel consumption of automobiles, a reduction in weight is intensively required in the automobile industry. For this reduction, the application of high-strength and ultra-high-strength steel sheets to automotive parts increases. The ultra-high-strength steel sheet, having a tensile strength above 1 GPa, is attractive for automotive parts. However, in cold stamping of ultra-high-strength steel sheets, large forming loads [1], large springback [2], small formability [3], tool failure [4], and hydrogen-induced delayed fractures [5] are problematic.

The shearing process of blanking and trimming is usually used in cold stamping of ultra-high-strength steel sheets. The shearing load for the ultra-high-strength steel sheets is increased by the large flow stress of the sheet [6]. The large shearing load brings tool wear and fractures. The effect of the die radius arc profile on the wear behavior was investigated [7], and then it was shown that tool wear and tool fractures in the trimming process of 980 MPa steel sheets are serious problems [8]. In trimming of the die-quenched steel sheets, the tool life is dramatically shortened [9]. A measurement method for chipping damage of the trimming tool for high-strength steel sheets was proposed [10]. On the other hand, to reduce the shearing load, the punch chamfer [11] and notch shear cutting [12] were proposed.

The qualities of the sheared edge in the steel sheets are important, and these steel sheets are often sensitive to sheared edge cracking and the magnetization properties. Basic shearing parameters such as the clearance and the incline angle on the sheared edge of 980 MPa ultra-high-strength steel sheets were investigated in the shearing process [13]. Yagita et al. [14] showed that the sheared edges of ultra-high-strength steel sheets are affected by the shear angle in the punch and the blank holding force. The sheared edge qualities for edge cracking in the hole expansion [15] and stretch flange abilities [16] were investigated. To improve the stretch flange ability of an ultra-high-strength steel sheet, smoothing of the sheared edge [17] was proposed. On the other hand, the magnetic properties of 20GN steel are affected by strain due to plastic deformation [18]. Weiss et al. [19] showed that mechanical stress and magnetostriction induced by shearing of non-orientated electrical steel reduced the magnetization properties. The magnetic properties of the electrical steel sheets used for electric motor parts were mainly investigated (i.e., the properties of the ultra-high-strength steel sheets were not investigated).

In the shearing process, a missing metal volume at the sheared edge appeared as dust and small pieces known as slivers. Slivers in trimming aluminum auto body sheets are problematic [20]. In shearing of aluminum auto body sheets, it was shown that the variations in cut surface quality and burr height are related to the cutting parameters of clearance, blade sharpness, and cutting angle [21]. Cracking from the tip of the sheared burr resulted in reduced elongation in tensile tests, but the effect on elongation was suppressed by using an appropriate clearance ratio [22]. During blanking of the aluminum sheets, particles are deposited as the picked-up material is scraped off, and the amount of particles easily varies with lubrication [23]. Furthermore, the crack behavior in shearing of aluminum auto body sheets was investigated by microscopic observation [24], and a numerical model of sliver formation was developed. Increasing the clearance ratio reduced damage to the sheared edge and suppressed the formation of slivers [25]. In the shearing process of ultra-high-strength steel sheets, the flying behavior [26] and the separation behavior of sheets having a curved shape [27] were reported. However, the scattering behavior of the dust and slivers are unclear in the shearing of ultra-high-strength steel sheets.

A schematic illustration of the auto body manufacturing process up to shearing is shown in Figure 1. The coiled rolled-steel sheet is handled by an electromagnetic chuck, which magnetizes the steel sheet. The uncoiled and leveled steel sheet is sheared to an appropriate size, and fine shearing slivers are generated in shearing. When the magnetized steel sheets are sheared, the shearing slivers magnetically stick to the sheets and tools, and the tools in the subsequent stages are worn by biting into the shearing slivers.

In this paper, the changes in the magnetization properties of high-strength steel and ultra-high-strength steel sheets are investigated, and then the edges and scattering behavior of slivers in sharing of the ultra-high-strength steel sheets are observed.

2. Materials and Methods

The mechanical properties of the sheet used are shown in

Table 1. Mechanical properties of steel sheets.

Steel Sheet	Thickness (mm)	Tensile Strength (MPa)	Elongation (%)	Reduction in Area (%)	n-Value
1180 MPa	1.20	1209	8.0	40.5	0.135
980 MPa	1.21	1029	15.7	45.0	0.133
780 MPa	1.23	799	20.0	57.5	0.125
590 MPa	1.21	599	23.2	58.3	0.152

. Sheets with nominal tensile strengths of 590 MPa, 780 MPa, 980 MPa, and 1180 MPa were used. All the steel sheets were dual-phase steel sheets, where the 590 MPa, 780 MPa, and 980 MPa sheets were galvanized alloy zinc (GA) steel sheets and the 1180 MPa sheets were non-coated sheets. The nominal thickness of all the sheets was 1.2 mm, and the elongation and reduction in area decreased with the increasing tensile strength of the sheets.

The percentage of content of the steel sheets is shown in

Table 2. Percentage of content of steel sheets.

Steel Sheet	Percentage of Content (Mass%)
	C	Al	Si	Mn	Zn	Fe
1180 MPa	2.19	0.00	0.51	2.82	0.00	94.48
980 MPa	3.29	0.44	0.11	2.49	0.82	92.85
780 MPa	2.22	0.46	0.19	2.38	0.67	94.07
590 MPa	4.27	0.00	0.39	1.60	1.76	91.98

. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed using an SEM (JEOL, JSM-IT100).

The method of investigating the magnetic flux density of magnetized and demagnetized sheets before and after shearing is shown in Figure 2. Blanks were cut to 50 × 70 mm by laser cutting to eliminate the influence of the cutting surface. The blanks were demagnetized with a demagnetizer KMD-20C (KANETEC, Japan) and then magnetized with a magnet of the same size as the blanks, with a magnetic flux density of 140 mT. The blanks that were simply demagnetized were used. The acceptability of demagnetization was guaranteed by a maximum flux density of less than 1 mT using a Tesla meter TM-801 (KANETEC, Japan). Each blank was measured for magnetic flux density distribution using the Tesla meter. The magnetic flux density distribution was measured in 5 mm intervals vertically and horizontally from the corners of the blanks. The blanks were then sheared by a die mounted on a servo press SDE-8018 (AMADA, Japan). The punch speed was 90 mm/s, the shear angle of the punch was 0 degrees, and the clearance ratio for the sheet thickness was 10%. The sheared sheets were again measured for the magnetic flux density distribution by the Tesla meter.

3. Results

3.1. Sheared Edge and Microstructures of the Steel Sheets

The sheared edges of the cut steel sheet are shown in Figure 3, and the quality of the sheared edge of each cut sheet is summarized in Figure 4. The sheared edge consisted of the rollover, the burnished surface, the fracture surface, and the burr. As the tensile strength of the steel sheet increased, the ratio of the burnished surface became smaller, and that of the fracture surface became larger.

The microstructures of the blank are shown in Figure 5. The surface microstructures of the sheets were observed using scanning electron microscopy. The microstructures became finer as the tensile strength increased. Specifically, needle-like structures were observed on the 1180 MPa steel sheet.

3.2. Magnetization Properties of the Steel Sheets

The relationship of the maximum magnetic flux density of the sheets and the magnetic flux density of the magnet used for magnetization is shown in Figure 6. The plots show the average of five iterations, and the error bars show the maximum and minimum values. The maximum flux density of the blank also increased as the magnetic flux density of the magnet increased, but the maximum flux density of the blank was saturated at a magnetic flux density of about 60 mT. As the tensile strength of the sheet increased, the maximum flux density after saturation also increased.

The magnetic flux density distributions before and after shearing of the 1180 MPa steel sheet, magnetized and demagnetized, are shown in Figure 7. The magnetized blank was magnetized with the north pole on the right and the south pole on the left, corresponding to the magnet used for magnetization, and the maximum magnetic flux density was about 15 mT regardless of the poles. When the magnetized blank was sheared, the magnetic flux density distribution was maintained, but the maximum flux density became smaller. In the demagnetized blank, the magnetic flux density distribution became random, and the maximum flux density became less than 0.5 mT. When the demagnetized blank was sheared, the flux density distribution remained as irregular as before, but the flux density increased at the corners.

The effect of the tensile strength of the steel sheet on the maximum flux density is shown in Figure 8. The plots show the average of five iterations, and the error bars show the maximum and minimum values. In the magnetized blank, the maximum flux density increased as the tensile strength increased. The sheared magnetized blank reduced the maximum flux density, but the larger the tensile strength, the larger the change. For the demagnetized blanks, the maximum flux density decreased to less than 1 mT regardless of the tensile strength. When the demagnetized blank was sheared, the maximum flux density increased, but the maximum flux density was less than 2 mT.

3.3. Scattering Behavior of the Shearing Slivers

The behavior of the shearing slivers at 980 MPa is shown in Figure 9, and the shearing load–punch stroke curve is shown in Figure 10. The fine shearing slivers generated during shearing were significantly dispersed when the fracture surface was penetrated and the blank fell off, as well as when the punch rose and came into contact with the cut surface. The reduction of the punch load matched that of the blank dropout. When the punch made contact with the burnished surface, large steel slivers scattered and fell off at high speed.

The measuring method of the mass of the shearing slivers generated per shearing shot is shown in Figure 11. A laser-cut 60 mm × 80 mm blank was mass-measured after demagnetization and then sheared at 20 mm from the top edge. The mass was then measured again after thorough cleaning and drying, and the change in mass was defined as the mass of the shearing slivers.

The effect of the tensile strength of the steel sheet on the mass of the shearing slivers is shown in Figure 12. The plots show the average of five iterations, and the error bars show the maximum and minimum values. The mass of the shearing slivers increased with an increasing tensile strength, with approximately 3.5 mg of shearing slivers being generated for the 1180 MPa steel sheet. The observation of the shearing slivers generated on the 1180 MPa steel sheet revealed sharp-edged particles with a width of approximately 10 μm.

The conditions for measuring the scattering mass of the shearing slivers during continuous shearing are shown in Figure 13. The sheared sample of the 1180 MPa steel sheet was used because it had the highest maximum flux density when magnetized and the highest mass of shearing slivers. The blanks 60 mm in width were magnetized and demagnetized, and then they were sheared continuously 10 times at a length of 10 mm. The mass of the shearing slivers was measured at the top and bottom of the die and at the blank.

The sheared surface and the scattering mass of the shearing slivers of the sheared 1180 MPa steel sheet are shown in Figure 14. The plots show the average of three iterations, and the error bars show the maximum and minimum values. The mass of the shearing slivers on the magnetized blank was about two-thirds that of the mass generated per shot, and the mass scattered to the top and bottom of the die was small. On the shearing surface of the magnetized blank, shearing slivers stuck to the blank were observed. On the other hand, the mass in the demagnetized blanks scattered to the bottom of the die was the largest, and about two-thirds of the shearing slivers generated were scattered to the outside. Thus, the mass of the shearing slivers on the blank was much less than the mass on the magnetized blank.

4. Discussion

A magnetostriction effect may be the cause of the change in the maximum magnetic flux density due to shearing. In the magnetized blank, as shown in Figure 7a, the magnetic domains were aligned in a single direction due to magnetization, and the maximum flux density was reduced when these were disturbed by strains due to shearing. On the other hand, in the demagnetized blank, as shown in Figure 7b, the magnetic domains were scattered by demagnetization, but they were locally aligned in a single direction due to strain by shearing, resulting in the increase in the maximum magnetic flux density shown in Figure 8.

As the tensile strength of the blanks increased, the mass of the shearing slivers generated increased, as shown in Figure 12, due to a decrease in ductility as shown in Table 1. The shearing slivers were scattered when the crack was penetrated and when the fracture surface made contact with the punch, as shown in Figure 9. The fracture surface was formed by the connection of cracks generated from the top and bottom of the blank. The high tensile strength of the steel sheet resulted in a high shearing load and instantaneous release of energy, resulting in heavily scattered slivers with a greater mass.

5. Conclusions

In this paper, the effect of the tensile strength of the steel sheet on the magnetization and the mass generated by the shearing slivers was investigated with the following results:

(1)

The maximum flux density in the magnetized blank increased with the increasing tensile strength of the steel sheet.

(2)

When the magnetized blank was sheared, the maximum flux density decreased but only by about two-thirds at most. The maximum flux density of the demagnetized blank decreased to less than 1 mT regardless of the tensile strength, and the maximum flux density increased after shearing, but the maximum flux density was less than 2 mT.

(3)

The shearing slivers were significantly generated at the time of crack penetration and at the time of punch rise, and their mass increased with the increasing tensile strength of the steel sheet.

(4)

In the shearing of the magnetized blank, the amount of shearing slivers sticking to the blank increased. This indicates that shearing of the magnetized ultra-high tensile steel sheets may cause die wearing in the subsequent stages due to the stuck shearing slivers.

(5)

When demagnetized steel sheets are sheared, shearing slivers do not adhere easily to the sheets, and the level of magnetization caused by shearing is not sufficient for the shearing slivers to adhere. Therefore, demagnetization before shearing is effective in reducing die wearing caused by shearing slivers.