Advanced high strength steels (AHSS) with duplex microstructures of ferrite and austenite will be developed analogous to high strength and fracture resistant titanium alloys that have acicular microstructures. The project team will address issues of current scientific importance with regard to bainitic phase transformations, phase transformation toughening and direct calculations of the physical properties of iron solid solutions. Systematic calculations of the electronic structure and crystal lattice distortions will be performed using the highly precise first-principles full potential linearized augmented plane wave (FLAPW) method. Theoretical calculations will examine atomic clustering and the effect of alloying elements on the drift of light impurities (C, N, and B). Full understanding of the atomic level properties will allow us to design steels with acicular microstructures on a smaller scale with greater strength and toughness.
The advanced high strength steels under investigation will facilitate the design of lighter weight vehicles that are more fuel efficient, but maintain passenger safety with improved crash worthiness. These new steels will keep steel competitive both in performance and cost with respect to aluminum and other lightweight materials. This program will help recruit and train individuals to work in the steel industry. The steel industry has always been an avenue of social change. Historically many ?new? Americans gained entrance into the middle class through this industry. We expect to continue this tradition and under-represented groups will be benefactors of this research either directly as student research assistants or indirectly through employment in the steel industry.
Interest to develop new advanced high strength steel for automotive applications is a result of increasing CAFE (corporate average fuel economy) standards where by 2025 an average fuel economy of 54.5 mpg is expected. Automotive steels will need to have ultimate tensile strengths greater than 900 MPa to remain competitive with magnesium and aluminum when these automotive materials are compared on a strength to density (specific strength) basis. It should be noted, however, that steelmaking contributes less CO2 to the environment (by a factor of 10 per ton of metal) than the manufacturing of either aluminum or magnesium. Furthermore, steels are stiffer and absorb greater energy in collisions and thus make vehicles safer. Development of third generation advanced high strength steels (AHSS) has been initiated to find lower cost steels than the austenitic second generation AHSS, but with properties exceeding those of first generation AHSS that are dual phase or martensitic. Target properties considered break-through for these new automotive steels would be combinations of ultimate tensile strength and elongation to failure of 1000 MPa and 30% or 1500 MPa and 20%. The attached Figure 1 shows the targeted property space for third generation steels and located in that diagram are all of the steels investigated during this research. Steel contains many interesting crystal structures and alloy composition plays an important role in determining which crystal structure is stable. Figure 2 shows the casting of an experimental steel in the Missouri S&T foundry. Two approaches can be used to increase the specific strength of steel: increasing strength to contain larger proportions of martensite and decreasing density through the addition of aluminum. Dual phase steels with strengths greater than 900 MPa have been achieved by increasing the volume fraction of martensite and austenite (MA). Addition of aluminum to lower the density of steel is also well documented. The aluminum atom is 12.6% larger than that of iron and 48% the atomic mass of iron. Thus, steel can be lightweighted by a combination of lattice dilatation and mass reduction. A physics based approach to alloy design was pursued to understand the role of Mn and Al in the defect formation with interstitial atoms and the stabilization of austenite. Systematic calculations of the electronic structure and lattice distortions are being performed using the highly precise first-principles full potential linearized augmented plane wave method (FLAPW) with the structure optimization capabilities. The NSF funded first principle studies are providing significant insight as to the role of Mn and Al alloying in steel. The result has produced two steels that meet third generation steels: a new class of steel with two mechanisms of transformation induced plasticity and a new duplex steel with acicular ferrite. Transformation induced plasticity (TRIP) behavior was studied in steel with composition Fe-0.07C-2.85Si-15.3Mn-2.4Al-0.017N that exhibited a maximum work hardening exponent of 1.4. The work hardening behavior was outstanding due to the high fraction of epsilon-martensite formed. The high strain hardening rate led to an ultimate strength of 1165 MPa at a necking strain of 33% and transformation of austenite to martensite (austenite to epsilon-martensite to alpha-martensite) led to enhanced elongation. In the second steel (Fe-13.92Mn-4.53Al-1.28Si-0.11C) a bainitic and acicular ferrite microstructure was observed and the prior austenite had a grain diameter of 16.5 µm, which was below the previously reported critical austenite diameter for acicular ferrite formation. The strong correlation between ferrite plate density and inclusions of galaxite and manganese oxide support a conclusion that acicular ferrite can be formed in small grained austenitic structures that might be produced for automotive applications. Upon tensile testing the retained austenite transformed to alpha-martensite. Ultimate tensile strength and elongation were 970 MPa and 40%. This study shows that an acicular, ferritic steel can be formulated where nonmetallic inclusions can be precipitated by thermal processing rather than formation during solidification. Acoustic emission was used here to study melting and solidification of embedded indium particles in the size range of 0.2 to 3 microns in diameter and to show that dislocation generation occurs in the aluminum matrix to accommodate a 2.5% volume change. To our knowledge, we are the first to report evidence of acoustic emission for the melting of embedded particles in a constraining solid. Any volume change associated with a diffusion controlled phase transformation may generate AE provided the relaxation of the product or parent phase occurs in less than 105 seconds. Comparisons to continuously cooled bainite suggest that acoustic emission should not be used as a criterion of displacive phase transformations. Strain energy may be as important as surface energy in terms of classical nucleation theory and dislocation generation at grain boundaries may explain the preference in ferrite nucleation along prior austenite grain boundaries in steel.
