A Study on Microstructure-Dependent Deformation and Failure Properties of Boron Alloyed Steel

Abstract: Developments in the automotive industry are driven by customer desires and legislative authorities. Legislation has restricted the emissions standards for vehicles, and has mandated the need for higher safety standards. The emission of carbon dioxide is directly related to fuel consumption, and the reduction in fuel consumption can be achieved by reducing the vehicle mass.A variety of methods have been used to reduce a vehicle’s mass while maintaining its crashworthiness. A technique using low-alloyed boron steel has been developed, and it enables the design of lighter body-in-white, while maintaining passenger safety. The technique is called press-hardening or hot stamping, and it involves the simultaneous forming and quenching of sheet metal. Press-hardened components have superior material properties compared to components made of mild steel. Another feature of compo-nents formed at elevated temperatures is the possibility of tailoring material properties in desired regions of the component. This is realized by using specially designed tools that allow differential in-die cooling rates and thus direct control of the formed microstructure. Using this technique, it is possible to manufacture a high-strength region next to a high-ductility section divided by a transition zone of mixed microstructure.The present work aims to determine the influence of mixed microstructures on the mechanical properties of low-alloyed boron steel. An experimental heat-treatment process is used to form multi-phase microstructures with a variety of phase volume fractions present in the composite. Digital image correlation is used to investigate the deformation of tensile specimens under loading. This full-field technique and a suitable constitutive model enables us to evaluate the flow and fracture properties of heat-treated samples. Microstructural characterization is used to determine the type of phases present and their average volume fraction in the composites.The findings from experimental studies are compared to results predicted by a constitutive model. A modeling strategy is employed to determine the effective material properties depending on the properties of single-phase characteristics. Failure of the material is indicated by stress-based fracture criteria. Numerical issues in finite-element modeling concerning the mesh-size sensitivity are addressed using a regularization method.The results of the experimental work aids the calibration and validation of the proposed microstructure-based modeling approach, and a knowledge of the processing history enables the prediction of the overall hardening behavior and fracture elongation. A comparison of experimental results, which are not used for calibration, with numerical results shows that there is good agreement.