Microstructures of hot-rolled high-strength steels with significant differences in edge formability

The relationship between microstructure and hole expansion was investigated for three industrial mill-processed steels with similar yield strength (about 525 MPa) and total elongation (about 25 pet). The nominal steel composition was (in mass pet) 0.1C, 1.4Mn, 0.1Si, 0.02Al, 0.04Nb, and 0.02Ti; any variations in composition or processing history were unintentional. The microstructures of all steels consisted of about 80 pet of proeutectoid ferrite and 20 pet of a carbon-enriched, hi,oh-hardness, low-temperature transformation product (LTTP). Despite these similarities, the hole-expansion values for the steels were 44, 74, and 115 pet. Detailed microstructural characterization revealed significant differences in the LTTPs of the three steels, as well as several important differences in the proeutectoid ferrite grains. Previously reported negative effects of large quantities of martensite, microstructural banding, and a high hardness ratio (LTTP/ferrite) were validated. Different hardness ratios correlated with differences in (1) dislocation substructures of proeutectoid ferrite grains, (2) grain-size distribution, and (3) the fine structure of bainitelike/pearlitelike regions. Superior hole-expansion performance (or edge formability) was associated with a microstructure consisting of 78 pet of uniformly fine-grained proeutectoid ferrite and 22 pet of a bainitelike microconstituent, a minimum amount of microstructural banding, and a low hardness ratio. Tensile-bar fracture surfaces of a material with this microstructure showed the largest amount of microplasticity.