Influence of Boron on the Flow Behavior of Hot Stamping Steel under Various Warm Working Conditions

Warm stamping of boron containing hot stamping grade steels is generally carried out at temperatures below austenitization temperature to reduce the forming load and spring-back. It also minimizes surface oxidation of steels which is prevalent during hot stamping process (forming at temperatures above austenitization temperatures). However, there is a possibility of occurrence of dynamic strain aging (DSA) at warm stamping conditions due to the pinning of dislocations by solute atoms such as boron which in turn can increase forming loads. This work investigates the influence of boron on the flow behavior in hot stamping steels under various warm working temperatures (300-773 K) and strain rates (10–2-10–4 s–1). Results indicate the occurrence of DSA in the temperature range of 473-573 K. The ultimate tensile strength (UTS) is found to increase and then decrease with an increase in the temperature due to a combined effect of dynamic strain aging and precipitation strengthening. The increase in the UTS by DSA is higher for steel with higher boron content (35 ppm) than steel with lower boron content (29 ppm). The formation of CrFeMn(CN) during warm stamping along with solid solution boron contributes to the dynamic strain aging.