Minimisation of Heating Time for Full Hardening in Hot Stamping Using Direct Resistance Heating

Tomoyoshi Maeno,Yoshitaka Nakao,Ken-Ichiro Mori,Ali Talebi-Anaraki,Masato Sakagami

doi:10.3390/jmmp4030080

Abstract

To obtain enough hardness of the die-quenched products after hot stamping using direct resistance heating, the effects of the electrifying condition and initial microstructure of the quenchable steel sheet on hardness were examined in a hot bending experiment. The steel sheet was heated up to 900 °C in 3 to 10 s. The required heating time was shortened by normalising heat treatment due to the fine grain size of the sheet. The standard deviation of the hardness of the sheet heated to 900 °C in 3.2 s without temperature holding at the austenitising temperature was 12 HV, whereas the deviation reduced to 5 HV for temperature holding at the austenitising temperature of 3 s.

Highlights

To reduce the weight and improve the crash safety of automobiles, hot stamping of quenchable steel sheets is useful
The stamped parts are hardened by quenching with dies, and ultra-high strength steel parts with a tensile strength of approximately 1.5 GPa are obtained under a low forming load
The quenchable steel sheets are heated to approximately 900 ◦C to transform the sheets to the austenite phase during heating [2]

Summary

Introduction

To reduce the weight and improve the crash safety of automobiles, hot stamping of quenchable steel sheets is useful. Mori et al [14] developed smart hot stamping of ultra-high strength steel products using rapid resistance heating and mechanical servo press. Löbbe et al [15] examined the influence of various austenitisation parameters such as rapid heating and cooling on the mechanical properties in hot stamping of ultra-high strength steel sheets. Mori et al [16] investigated the warm stamping of ultra-high strength steel sheets using rapid resistance heating around a heating temperature of 300 ◦C. Maeno et al [25] improved and shortened the holding time for die quenching by the water and die quenching process in the hot deep drawing test They reduced the holding time for hardening due to the high cooling speed. 2a.tHthoet aSutasmtenpiitnisginUgstienmg pDeirraetcutrRe ewsaisstapnecrfeoHrmeaetdining hot stamping using resistance heating

Hot Stamping Having Simultaneous Heating of Next Blank with the Die Quenching

Conclusions