Original Article

Numerical simulation of hydrogen absorption during continuous annealing of hot dip galvanized steels

ArcelorMittal Global R&D Maizières Research, Voie Romaine, BP30320, 57283 Maizières-Lès-Metz, France

^* e-mail: thomas.dieudonne@arcelormittal.com

Received: 24 June 2024
Accepted: 12 December 2024

Abstract

AHSS grades present a good resistance/formability compromise to produce structural and safety parts for automotive. In order to improve their resistance to corrosion, these steels can be coated with a zinc layer by hot dip galvanization. After cold rolling, the steel strip is annealed under controlled atmosphere and temperature according to a well-defined thermal cycle to obtain the targeted microstructure, then it is immersed in a liquid zinc bath at 460 °C. The annealing performed under reducing atmosphere composed of N₂ + 5% H₂ induces a significant absorption of atomic hydrogen in the bulk. At the exit of the galvanizing bath, the zinc layer in surface of the strip constitutes a barrier to the hydrogen desorption during the cooling and the storage of the coil at room temperature before cutting and stamping by the customer. This hydrogen absorbed during the process can be harmful for the zinc coated steel grade by inducing a deterioration of its formability or delayed fracture phenomena after cold stamping. It is then important to control this absorbed hydrogen and to be able to predict the content introduced in the steel as a function of the annealing parameters and the evolution of microstructure during this annealing. A numerical model was developed by FEM simulation to predict the hydrogen content absorbed in the steel during the galvanization annealing as a function of annealing parameters and microstructure. This physical model uses Sievert and Fick’s laws to estimate the solubility and the diffusivity of hydrogen in the steel as a function of the annealing temperature. In addition, the influence of trapping by microstructural defects in the different phases during the annealing is described in the model by McNabb and Foster’s approach.