Abstract
Steel sheets of Fe-0.2C-2Mn-0.2Si-0.03Ti-0.003B (m%) for the automotive industry were isothermally heat-treated, comprising austenitizing and subsequent isothermal annealing at temperatures between 300°C and 500°C. As a consequence, microstructures ranging from granular bainite over lower bainite to auto-tempered and untempered martensite were obtained. In tensile, hole expansion and bending tests, the performances in different forming conditions were compared and the changes of microstructure and texture were studied by complementary electron backscatter diffraction (EBSD) analyses. Samples with granular bainitic microstructures exhibited high total elongations but lower hole expansion ratios; in subsequent EBSD and texture analyses, evidence for inhomogeneous deformation was found. In contrast, the lath-like bainitic/martensitic microstructure showed higher strength and lower elongation to fracture. This results in a reduced bendability, but also in a high tolerance against damage induced by the shearing of edges, and, thus, allows homogeneous deformation to higher strains in the hole expansion test.
Highlights
Cold-forming operations of advanced highstrength steels (AHSS) play an important role in applying lightweight design to automotive vehicles
The effect of the morphological differences in isothermally heat-treated Fe-0.2C2Mn-0.2Si-0.03Ti-0.003B (m%) sheets on cold deformation was investigated and the following conclusions can be drawn: Isothermal heat treatments at temperatures between 500°C and 425°C lead to a microstructure consisting of granular bainite with columnar aligned M–A islands
A reduction of Tiso down to 375°C leads to a predominant constituent of bainite
Summary
Cold-forming operations of advanced highstrength steels (AHSS) play an important role in applying lightweight design to automotive vehicles. A variation of microstructural constituents is realized by applying different isothermal transformation temperatures. This allows for a comparison of the suitability of individual constituents for different forming operations. The homogeneity and the anisotropy of the material, resulting from the rolling and annealing process, has to be considered.[2,3,4] Especially in the outer surface, hardness differences in the individual phases, and the morphology as well as the initial texture, are critical for the bendability.[5,6] Hole expansion tests (HET) allow the evaluation of the stability of punched edges. Coarse grains and significant strength differences in the microstructural constituents have been reported to deteriorate the performance in the HET.[7,8,9]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
