Elevated Fracture Resistance of Q&P1180 Steel after Forming and Paint-Baking

<div class="section abstract"><div class="htmlview paragraph">According to several precedent studies, most of the cold-forming advanced high-strength steel (AHSS) grades can obtain reinforced yield strength from the automotive forming and paint-baking treatments without losing their fracture resistance like some aluminum alloys. Concisely, the mechanism of such behavior can be mainly attributed to the ‘Cottrell Atmospheres,’ some thermally mobilized interstitial atoms that cluster around and impede mobile dislocations during only the yielding stage of the plastic deformation but cannot continue durably enough to affect the fracture. Nevertheless, an exception, Q&P1180, was discovered from precedent studies and characterized in this work. Different from other AHSSs, this grade exhibited distinctively elevated fracture resistance and yield strength after the pre-straining and baking. Such uniqueness was speculated to be caused by 1) no soft ferrite in the microstructure and 2) the transformed fresh martensite induced by the plastic deformation being tempered-softened by the paint-baking cycle. Such two microscopic features narrowed the micro-hardness differences among the neighboring multi-phases in the deformed and baked Q&P1180 microstructures, which consequently retarded the micro-voids creation and coalescence to the macroscopic fracture. To support this speculation, a series of mechanical experiments were conducted, including various treatment conditions, stress-state, and strain-rate dependencies, on the pre-strained and baked Q&P1180 samples. As a comparison, other multi-phase AHSS grades, such as Q&P980 and DP980LCE, were also tested to indicate their lack of such behavior. Eventually, the test results characterized the consistently elevated fracture resistance of Q&P1180 under all the investigated conditions and accordingly highlighted this target material’s advantageous crashworthiness in the automotive applications.</div></div>