Microstructure Transformations in a Press Hardening Steel during Tailored Thermomechanical Processing

Abstract

Microstructure transformation due to thermomechanical processing has an acute effect the macroscopic properties of low‐carbon steels. This effect includes viscoplastic deformation and phase transformations. Hot forming processes such as press hardening are particularly affected. Most engineering applications require a combination of high strength and sufficient residual ductility, which can be achieved by the development of graded microstructures. Herein, the evolution of phase transformations is investigated by linking experiments and simulations to produce graded microstructures. For this purpose, an extended material model is proposed to represent the evolution of phase transformations under inhomogeneous heating and cooling strategies. On the experimental side, phase transformations are identified during thermomechanical treatment of flat steel specimens using digital image correlation and thermal imaging. Based on the experimental results, the material parameters are identified, and the simulation model is validated. On the numerical side an algorithm for the finite‐element simulation of phase transformations in low‐carbon steels is proposed. The evolution of phase transformations is presented for the simulation of a tensile specimen using the finite‐element method.

Constitutive modeling of viscoplasticity including phase transformations for graded thermo‐mechanical processing

“…The validity of the dissipations in Eq. (3) and thus thermodynamic consistency is proven in more detail in [2]. M is the Mandel stress tensor and L i represents the inelastic part of the velocity tensor.…”

“…The hardening stresses Q 1 and Q 2 as well as the chemical forces Z i are defined conjugate to their internal variables q 1 and q 2 as well as z i , respectively. The evolution equations describing the internal variables z i directly depend on the temperature and are derived partly in [1] as well as in [2].…”

“…In accordance with the constitutive equations of the proposed prototype model in [2], two numerical examples for phase transformations are depicted. The first example considers a uniaxial loading case over a total time of 370 s, with austenitization temperatures between θ = 800 • C and θ = 900 • C, a cooling rate of 14 K/s as well as a total uniaxial strain of ε = 20 %.…”

Microstructural Evolution of a Hot-Stamped Boron Steel Automotive Part and Its Influence on Corrosion Properties and Tempering Behavior

“…However, due to the variation of deformation and heat extraction conditions along the intricate geometry of these components, certain regions can face incomplete martensitic transformation or strain-induced ferritic and bainitic transformations, presenting complex microstructures consisting in mixtures of martensite, bainite and/or ferrite. Such microstructural complexity has been somewhat addressed in works usually involving hot tensile testing at different temperatures, in attempts to understand the effects of thermomechanical parameters on phase evolution 4,5 . Even studies focused on machine learning approaches to predict such microstructure have been recently developed 6 .…”