Non-isothermal forming limits of press-hardening steels during hot stamping

Abstract

The development of modern lightweight vehicles necessitates the use of press-hardening steels (PHSs) that can enhance occupant safety while reducing fuel consumption due to their exceptional strength and energy-absorption capabilities. The objective of this study is to develop a methodology to determine the hot formability of PHSs during non-isothermal hot-stamping processes and to gain insights that can be broadly applied across different material systems. The work is focused on an Al-Si coated PHS grade with 1800 MPa tensile strength in the hot-formed condition, designated as PHS1800. The hot formability tests were performed via the Marciniak test setup with a quenching carrier blank using in-situ stereo digital image correlation (DIC) strain measurements. The dependence of formability on the hot-stamping conditions was systematically examined by altering the initial forming temperature, punch speed, and cooling rate within the ranges of 600–750 °C, 10–40 mm/s, and 20–50 °C/s, respectively. To predict the non-isothermal forming-limit curves (FLCs), a numerical modeling scheme based on the Marciniak-Kuczyński (MK) theory was established that incorporates the evolution of temperature and strain rate during hot forming. A process-dependent function was proposed for the initial imperfection factor to predict the FLCs for a wide range of hot-stamping conditions beyond those considered in the model calibration. The Marciniak test procedure provided approximately linear strain paths within a strain-state range from uniaxial drawing to equibiaxial stretching. The measured limit strains revealed that the formability increases with the initial forming temperature and is reduced with increases in the forming speed and cooling rate. The predicted FLCs were in close agreement with the measured limit strains as functions of the initial forming temperature, speed, and cooling rate. The developed numerical scheme provides a predictive tool to calculate variations in the non-isothermal limit strains of press-hardening steels during elevated-temperature forming, which is crucial for optimizing tooling design and process parameters.