Effect of Two Steps Overaging on Mechanical Properties of Tailor Rolled Blank of Dual Phase Steel

Herein, a new kind of overaging strategy: two steps of overaging for tailor rolled blank of dual phase steel (DP-TRB) was investigated. The results indicate that two steps of the overaging process is a useful way to control the mechanical properties of DP-TRB. In the premise of satisfying the requirement for the strength of DP590 grade, the total elongation can be significantly increased (3~7% in most cases). Due to the different degrees of ferrite recrystallization (differences of densities of dislocation) among the thicknesses, the obvious changes of mechanical properties among thicknesses are found. The thicknesses zones of 1.0~1.4 mm show lower strength, while the zones of 1.6~1.8 mm present higher strength. Otherwise, the high density of dislocations in samples of 1.6~1.8 mm provide more locations for Cottrell atmospheres, yield plateau occur easier. The zones with different thicknesses for one DP-TRB show two kinds of yield behaviors (continuous yield and non-continuous yield) simultaneously. The subtle C diffusion control by two step overaging leads to the quite different pinning effect of Cottrell atmospheres. Thus, the pinning effect occurs in a gradual way, and a transition state of yield behavior, which combines the characteristic of smooth curve in continuous yield and the plateau in non-continuous yield, is found.

A Method for Evaluating the Formability of Tailor Rolled Blank (TRB) by Means of the Forming Limit Margin Field Graph in Forming Process

Tailor rolled blank (TRB) with graded thickness has shown great potential in the automobile field. Using traditional forming limit diagrams (FLDs) to evaluate TRB formability is challenging due to thickness variations. In this paper, a 3D forming limit surface (FLS) considering the influence of thickness was obtained. A numerical model was developed to predict final strains. Moreover, a forming margin was denoted and calculated to generate the forming limit margin field graph for quantitative evaluation of the TRB formability. Results showed that as the punch travel increased, the forming margin value decreased. As the travel changed from 35.2 mm to 37.4 mm, the corresponding forming margin value changed from 0.002 to -0.024. The formability declined, and the specimen eventually cracked on the thinner side. Besides, the deformation and strain paths predicted by simulation agreed well with those measured from formed part, which indicated that the final strains used in formability evaluation were reliable. The method was suitable for quantitative evaluation of the formability and predicting the cracking position in TRB forming.