Passive Laser Assisted Bending of Ultra-High Strength Steels

Abstract

Formability of ultra-high strength steels is poor causing problems in bending and stretch forming. The target of this work was to improve the formability of ultra-high strength steel sheets by controlled local laser heat treatments. Three steel grades, a bainitic-martensitic 4 mm DQ960 and two martensitic WR500 with 6 mm and 10 mm thicknesses were heated by controlled thermal cycles using a 4 kW Yb:Yag –laser, followed by self-cooling. Sheets with the thicknesses of 4 and 6 mm were treated on one side only by heating up to the austenitizing temperature. The 10 mm thick WR500 sheet was heat treated separately on the both surfaces by heating to a lower temperature range to produce a shallow tempered layers. The tensile and bendability tests as well as hardness measurements indicated that laser heat treatment can be used to highly improve the bendability locally without significant strength losses.