Combination of Chemistry & Steel Mill Processes
NanoSteel technology can be produced in a typical commercial sheet steel production mill, using existing equipment, and does not require the addition of any special processing methods.
In the steel mill, the selected alloy is continuously cast into slabs using standard high-throughput mill equipment. Upon solidification, the steel contains dendritic structures with austenite grains and interdendritic pinning phases containing nanoprecipitates. The image of this structure, along with the further steps in the process development, is shown in the figure below.
In the next stage, the slabs are heated in the tunnel furnace and hot rolled to reduce thickness – simultaneously forming the Nanomodal Structure. The Nanomodal Structure is where significant grain size reduction takes place, leading to a substantial improvement in properties.
Next, the sheet is cold rolled, which also further develops its strength. The High Strength Nanomodal structure of the cold rolled sheet emerges as a result of extensive deformation through dislocation mechanisms with phase transformation. At this juncture, the structure consists of transformed nanoscale ferrite grains, retained austenite, and nanoprecipitates resulting in increased strength with an associated reduction in elongation.
At this point, the material must be annealed to restore ductility before it can be used for automotive structural components. This application of heat recrystallizes the alloy, removes the cold-work effects, and provides a phase transformation of the ferrite back to austenite. When the sheet with recrystallized structure deforms, it leads to formation of the Mixed Microconstituent structure providing the targeted property combination of tensile strength and elongation.
The following video further explains NanoSteel's AHSS performance capabilities:
VIDEO: Increased Formability with NanoSteel 1:46