A thermo-metallurgical-mechanical model for microstructure evolution in laser-assisted robotic roller forming of ultrahigh strength martensitic steel

Laser-assisted robotic roller forming (LRRF) apparatus and process were developed to bend a plate to form a straight channel for ultrahigh strength steel MS1300. Since the thermal processing during the roller forming impacts the microstructure and mechanical behavior of the steel, an integrated thermo-metallurgical-mechanical finite element simulation considering the heat source, phase transformation and material constitutive models was established. A rectangular laser source was devised to homogenize the temperature around the bending corner and a new surface heat source model was proposed and validated. The phase transformation model accounting for the austenitization process, austenite decomposition and tempering was embedded in the finite element model through self-developed user subroutines. The predicted microstructure evolution and the phase distribution were consistent with experimental microstructure characterization. More specifically, it is found that tempering dominates at the inner layer of the bend, resulting in two different phases, i.e., the original and tempered martensitic phases after the LRRF process. The outer layer of the bend, however, goes through austenitization, quenching, and tempering processes, resulting in a combination of fresh martensite, a small amount of tempered martensite and retained austenite phases.