Viscoplastic constitutive modeling of boron steel under large strain conditions and its application in hot semi-cutting

To overcome the long process time of laser cutting and the high costs for repairing cold blanking tools of ultrahigh strength hot stamping components, the hot semi-cutting process was proposed. The reliable testing and parameter identification method of the constitutive behavior under large strain conditions are essential for an accurate simulation of hot stamping and hot semi-cutting process. In the present work, a new testing and parameter identification method for the viscoplastic constitutive behavior of boron steel is proposed based on the Gleeble system, which includes a new grip design, shear specimen selection, and its relevant high-temperature and high-speed digital image correlation (DIC) technology. To obtain the stress-strain curves under large deformation, the FEM-based optimization method is also proposed to identify the hardening parameters. To cover a greater strain range, a new viscoplastic hardening model is proposed based on the dislocation density evolution under elevated temperature deformation conditions. The reliability of the proposed testing method and hardening model is verified by the analysis of uniformity of temperature and strain rate, and the comparison of predicted stress-strain curves with parameters identified from shear and tensile testing. The thermomechanical tension of the center-hole and notched specimen, and the hot semi-cutting process are carried out to verify the proposed testing method and hardening model. The results show that the shear specimen has a more uniform temperature and strain rate distribution in the deformation zone than the tensile specimen. Shear specimens can be used to test the hardening behavior under various process conditions until failure at the large strain. The insensitivity to phase transformations of shear testing is another advantage for identifying the constitutive behavior of the product phase formed during continuous cooling. The constitutive behavior of the boron steel under a large strain range can be accurately identified by using the FEM-based optimization method proposed in this paper. Besides, the mechanical response of the center-hole specimen, notched specimen, and the hot semicutting deformation predicted by the proposed hardening model shows accurate results.

Automated summary

In this study, a testing and parameter identification method for the viscoplastic constitutive behavior of boron steel based on the Gleeble system was proposed, along with an FEM-based optimization method to identify the hardening behavior under large deformation. Shear specimens were used for testing hardening behavior with more uniform temperature and strain rate distribution, and showed high accuracy in identifying the constitutive behavior of product phase. The proposed methods and models were verified through thermomechanical tension tests and hot semi-cutting processes, showing accurate results.