Funding provided by this grant will be used to develop a new generation of advanced high strength steel (AHSS) exhibiting exceptional mechanical properties achieved by creating microstructures comprised of martensite and austenite. Determining the maximum amount of austenite that can be retained in relatively lean alloys by fully exploiting the phenomenon of stabilization will be a central goal of the work. A double stabilization processing scheme will be employed. Candidate alloys will be fully austenitized after cold rolling and cooled rapidly enough to circumvent the bainite transformation in route to an initial hold temperature. The first stage of stabilization will occur at that point when the steel is briefly held just above its Ms temperature. The steel then will be cooled to its final quench temperature chosen to cause a prescribed volume fraction of the austenite to transform to martensite. Additional stabilization will occur when the steel is aged at a higher temperature to allow partitioning of carbon from the martensite into the austenite under paraequilibrium, carbide-free conditions. This second stabilization will enable the austenite to have the required resistance to transformation to martensite when the steel is finally cooled to room temperature.
The broader impact of the project is the development of a new generation of steel for advanced engineering designs. To realize a viable new structural material a high volume fraction of austenite with optimized stability must be obtained within the constraints of a carbon content low enough to not severely compromise weldability, alloy levels that do not greatly increase the cost of the steel and a processing path consistent with current sheet steel production practice. The strength and ductility combinations that will become available are intended for the production of lighter weight components with increased resistance to fracture enabling greater energy savings and margins of safety.
Driven by regulations legislating improved passenger safety and the impetus for rising fuel economy, the transformation industry is increasingly in demand of advanced high strength steel (AHSS) grades to manufacture its products. Our project under NSF support targeted the development of a third generation of AHSS with strength and ductility combinations exceeding those of ferritic first generation AHSS and that can be produced much more economically than austenitic second generation AHSS. The strength/ductility goal of a third generation AHSS can be achieved by employing a microstructure comprised of ferrite with martensite and a significant volume fraction of austenite. The key to a viable third generation AHSS is obtaining a high volume fraction of austenite within the constraints of a carbon content low enough to not severely comprise weldability, alloy levels that do not greatly increase the cost of the steel, and a processing route aligned with current sheet steel production practice. Using our dual stabilization thermal processing cycle we were able to achieve an austenite level of greater than 30 vol. pct. in a steel containing in wt. pct. 0.29 carbon, 3.99 manganese, 2.12 silicon and 1.50 aluminum. The balance of the alloy’s microstructure was comprised of 23 vol. pct. ferrite and nominally 47 vol. pct. martensite. The ferrite formed during the austenitization segment of the thermal cycle (900oC for 60 seconds). The resulting micron size dispersion of ferrite in a matrix of 77 vol. pct. austenite, now mostly transformed to martensite, is shown in Figure 1. As an example of a full dual stabilization thermal cycle is as follows. After stabilization the steel was quenched to 450oC for 5 seconds (the first stabilization event), then quenched to 140oC for 5 seconds to transform more than half of the 77 vol. pct. austenite to martensite followed by carbon partitioning at 450oC for 60 seconds (the second stabilization event) and finally forced-air cooled to room temperature. That thermal cycle produced an austenite content of 32 vol. pct. The austenite is typically found to be in the size range of 100 to 300 nm and located among packets of martensite laths as illustrated in the TEM micrograph displayed in Figure 2. Evidence that the austenite is uniformly distributed at the micron size scale is provided by the EBSD phase map contained in Figure 3. Even with 32 vol. pct. austenite the steel attained a tensile strength of 1,350 MPa based upon its measured Vickers hardness of 412. Bend tests revealed that the steel also possessed a high level of ductility.
