Prevention of delayed cracking of punched 1.5 GPa ultra-high strength steel sheets by ironing with punched slug

Ken-Ichiro Mori,Yusuke Murai,Yohei Abe

doi:10.1007/s00170-020-05786-w

Ken-Ichiro Mori, Yusuke Murai + Show 1 more

Open Access

https://doi.org/10.1007/s00170-020-05786-w

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Abstract

Hydrogen-induced delayed cracking of punched 1.5 GPa ultra-high strength steel sheets was prevented by ironing with a slug ejected from punching, because the risk of delayed cracking for sheared 1.5 GPa sheets is very high due to high tensile residual stress and large plastic deformation. To prevent the occurrence of delayed cracking, the sheet was punched, and then, the punched hole was ironed by passing the punched slug. Although the residual stress around the punched edge of the 1.5 GPa sheet was tensile, the stress was turned to compressive stress by slug ironing. In addition, the fracture surface of the sheared edge was changed to a smooth ironed surface. A cathode hydrogen charging test of the punched edge for delayed cracking was performed. Although delayed cracks were caused at the punched hole by hydrogen charging, no cracks occurred at the ironed hole for charging. It was found that slug ironing of the punched hole is effective in preventing delayed cracking.

Highlights

For the weight reduction and passenger safety of automobiles, the use of ultra-high strength steel sheets having a tensile strength above 1 GPa for body-in-white parts rapidly increases [1]
Hot stamping is major for production of automobile 1.5 GPa steel parts, cold stamping of the ultra-high strength steel sheets has the advantages of high productivity, conventional and cheaper equipment, no oxidation prevention treatment, etc
It was found that the elimination of the tensile residual stress by slug ironing is greatly effective in preventing the occurrence of delayed cracking, and punching for a clearance ratio of about 10% and slug ironing are useful in preventing delayed cracking of the 1.5 GPa steel sheets without a burr

Summary

Introduction

For the weight reduction and passenger safety of automobiles, the use of ultra-high strength steel sheets having a tensile strength above 1 GPa for body-in-white parts rapidly increases [1]. 480 VH1 order to smooth the sheared edge, and Kadarno et al [31] developed a punching process including thickening around a hole with a taper punch and step die In these punching processes, the tensile residual stress is considerably reduced by applying large shear deformation to the sheared edge under a high contact pressure, and the risk level of delayed cracking becomes low. In punched 1.5 GPa ultra-high strength steel sheets, the risk level of hydrogen-induced delayed fracture is very high due to high tensile residual stress and large shear deformation around the sheared edge. A process for preventing delayed cracking in punching of 1.5 GPa ultra-high strength steel sheets is proposed in this study Since steel sheets are conventionally punched at a high punching speed in industry, a punching speed of 200 mm/s was chosen

Results of punching and slug ironing

Results of delayed cracking

Combination of punching and slug ironing processes into one stage

Conclusions