A new coupled thermomechanical framework for modeling formability in transformation induced plasticity steels

• Published in: International journal of plasticity 2018-04, Vol.103, p.39-66
• Main Authors: Connolly, Daniel S., Kohar, Christopher P., Mishra, Raja K., Inal, Kaan
• Format: Article
• Language: English
• Subjects: Automobile industry; Axial stress; Boundaries; Conduction heating; Constitutive models; Deformation; Energy efficiency; Formability; Forming limit diagram; Fuel consumption; Hardening; Kinematics; Martensite; Martensitic transformations; MK analysis; Modelling; Phase transformation; Plane strain; Plastic properties; Retained austenite; Stability; Strain rate; Temperature dependence; Temperature effects; Thermal transformations; Thermomechanical analysis; Thermomechanical modeling; TRIP steel; TRIP steels; Weight reduction; Yield strength
• ISSN: 0749-6419; 1879-2154
• DOI: 10.1016/j.ijplas.2017.12.008

Transformation induced plasticity (TRIP) steels have significant volume fractions of retained austenite that can undergo a strain induced transformation into martensite. This transformation, known as the TRIP effect, produces a high hardening capacity that can lead to enhanced formability, which can result in weight reduction and improved vehicle fuel efficiency for automakers. In this paper, a phenomenological framework for TRIP steel is integrated into a Marciniak-Kuczynski (MK) model coupled with a thermal solver to create a new fully coupled thermomechanical formulation to evaluate formability. The constitutive model was calibrated to capture the kinematics of martensite and flow stress dependence on strain rate, temperature, triaxiality, and stress asymmetry for TRIP 800 steel. Several sensitivity and exploratory studies are performed to highlight critical mechanisms for modeling TRIP in formability. Kinematic effects of transformation are shown to have a minor effect on formability compared to the hardening and evolving yield surface effects. Thermal effects, such as conduction, convection, and radiation heat transfer, are shown to be crucial for the formability of TRIP 800 at elevated sheet temperatures with room temperature external boundaries, but not for elevated external boundaries. By modifying the sheet initial thermal conditions, martensite transformation could be controlled to be able to delay localization and enhance formability in the plane strain and uniaxial formability by 25% and 35% respectively. •Introduction of martensite transformation model into Marciniak-Kuczynski framework.•A new coupled thermomechanical Marciniak-Kuczynski framework.•First coupled thermomechanical simulation of TRIP formability.•Effects on formability of martensite transformation mechanisms.•Room and elevated temperature formability with thermal boundaries.•Formability enhancement of up to 35% for TRIP 800 by delaying martensite formation.