Rapid advances are being made in materials for ...



Results from Prior NSF Support

Note to reviewers: Items shown in red in this section are developments made with NSF support to the PI’s that make possible the proposed research project.

DMR 0139045: GOALI / Plastic Anisotropy and Bauschinger Effect: Fundamental Role of Second-Phase Particles, R. H. Wagoner, F. Barlat, 7/02-6/06, $500,000. [1-40]

Research (Intellectual Merit) (References shown by contributing individual below.)

• Developed and presented two mechanical tests: in-plane sheet tension/compression and simple shear tests (highest cited paper in Int. J. Plast. for 2005)

• Designed and constructed a springback draw-bend test machine

• Demonstrated that elastic models of the BE cannot capture its essence

• Established aging conditions and mechanical behavior for novel Al-Ge-Si alloys

• Linked measured BE effect to TEM-characterized precipitates in Al-Ge-Si alloys

• Introduced new finite element for implementation of advanced constitutive equations

Scholarship [References 1-40] (Broad Dissemination / Broader Impact)

• 23 publications in print: 16 peer-reviewed, 7 conference proceedings

• 5 Ph.D. dissertations completed, 2 book chapters accepted for publication

• 4 publications submitted or in press, 6 invited/international presentations

Human Resources Developed (Broader Impact)

• 5 Ph.D. students (graduated and employed., one female)

• 2 post-doctoral researchers, now employed (one assistant professor, one industry)

DMR 0303510: Inter-American Materials Collaboration – New Sheet Steels Developed Through Carbon Partitioning, J.G. Speer and D.K. Matlock, 8/03-7/06, $329,000.

Research (Intellectual Merit)

• Identified and confirmed basic principles for a fundamentally new heat treating process to produce high strength steel

• Developed a model, based on kinetic considerations, to predict processing conditions

• Developed a unique international (Brazil, England, Belgium, and Korea) team of investigators to pursue fundamental aspects of new heat treating process

• Evaluated applicability of process to potential bar, plate, and sheet products and to high carbon cast irons.

• Multiple companies from around the world are now evaluating the applicability of this new technology.

Scholarship (Broad Dissemination / Broader Impact)

• 18 publications in print: 10 peer-reviewed, 8 conference proceedings [1-22]

• 4 publications submitted and currently under review

• 1 Ph.D. dissertations and 1 MS thesis completed [23-24]

• >16 invited/international presentations [25-40]

Human Resources Developed (Broader Impact)

• 1 Ph.D. student (graduated and employed, female) {Dr. Amy Clarke will receive the prestigious “Willy Korf Award for Young Excellence” from the International Steel-Success-Strategy (SSS) Conf. New York June 2007}.

• 1 MS student (graduated and employed)

• 1 post-doctoral researcher, now employed (steel industry)

• 4 visiting overseas graduate students (one female; CSM faculty on thesis committees)

• Group project for CSM Senior design team (5 undergraduate students)

• 1 undergraduate research assistant.

INTRODUCTION

Rapid advances are being made in materials for transportation vehicles that reduce mass by increasing strength. Such materials offer many societal advantages: energy conservation, increased safety, and reduction of environmental impact. Advanced High Strength Steels (AHSS) offer amazing combinations of strength (for light weight, performance) and ductility (manufacturability). They are particularly promising for crash-resistant autobody structures, where stiffness, strength, and energy absorption are required in stamped parts. Fundamental technical questions must be addressed before AHSS can be used with confidence by manufacturers, thus allowing the potential benefits to be achieved.

Background: There is a growing need to conserve energy, reduce greenhouse gases and other emissions, increase safety, and improve performance of transportation vehicles. These goals require innovative solutions. Driven by the marketplace, regulation, and competition, a range of non-traditional materials have been increasingly adopted. In particular, revolutionary advances (and anticipated ones) in iron-based alloy metallurgy and mechanics are creating major opportunities for improving society while posing exceptional challenges in terms of fundamental knowledge of advanced alloys.

The major part of the mass of a typical automotive chassis has been, and continues to be, sheet-formed steel. Thus, sheet formability and springback are crucial aspects for these applications, along with many other considerations such as strength, corrosion resistance, and weldability. Within a given class of sheet steels, formability and strength usually vary inversely as shown in the mutli-colored region of Fig. 1.

|[pic] | |

| | |

| | |

| |Figure 1: Trade-offs of strength and ductility|

| |of traditional and advanced high-strength |

| |steels [1]. (Courtesy Auto/Steel Partnership.)|

Until about 1990, traditional steel grades used in automotive body structures relied principally on ferrite (bcc iron) and carbide (Fe3C, alloy carbides) metallurgy, about which much has been known for at least 50 years. Examples of such alloys include mild steels, IF (interstitial-free) steels, bake-hardenable steels, solid-solution strengthened steels, and HSLA (high-strength, low-alloy) steels.

The first generation of AHSS was introduced in the 1990’s and 2000’s, eventually replacing many older grades. The fundamental metallurgy and micro-mechanisms of these grades are well-established. Application issues, particularly forming and springback, remain critical concerns for wide adoption of all AHSS alloys. (These issues are addressed in Thrust 1 of this proposal.)

First generation AHSS retain a predominantly ferrite matrix, but rely on secondary structures of bainite, martensite, and metastable austenite to achieve high strength and concurrent high formability (which are usually mutually exclusive within one metallurgical system). Examples of first-generation AHSS include martensitic, dual-phase (DP) steels and TRIP grades, Fig. 1.

Second-generation AHSS are now being developed. They rely on a matrix of austenite (fcc iron). Large quantities of manganese are added to stabilize the fcc crystal structure, producing grades known as TWIP (twinning-induced plasticity) and L-IP. Exceptional strength and ductility are potentially achievable, Fig. 1, but the high cost of the alloying elements prevents widespread adoption of these grades.

Third-generation AHSS are envisioned as based on metallurgical aspects and micro-mechanisms that borrow from both first and second generation AHSS to achieve strengths and formabilities intermediate between the two, Fig. 1, at costs that make automotive adoption feasible. Development of candidate third-generation AHSS is the second technical thrust of the current proposal.

In addition to technical issues surrounding AHSS, education and training in this field has become a major obstacle. Colorado School of Mines (CSM) and the Ohio State University (OSU) have strong programs in, and commitments to, steel and sheet forming education. A third thrust of this proposal is attracting, and improving the education of, undergraduate and graduate students in these areas. The three proposal thrusts will proceed in parallel and will each be enhanced by close interactions.

Project Overview: The proposed collaborative project will address the most pressing technical issues for widespread adoption of existing AHSS, and development of affordable 3rd-generation AHSS. These issues are consistent with those identified by the NSF Workshop entitled Advanced High-Strength Steels: Fundamental Research Issues and held October 22-23, 2006 [2]. The first PI organized this workshop, which attracted a diverse group of 60 scientists and engineers, and the second PI presented one of three invited lectures summarizing the field.

The project is structured with three thrusts:

1. Process Development: formability and springback of existing and new AHSS

2. Material Development: identification of new processing routes and microstructures upon which to base a 3rd generation of AHSS

3. Educational Program: attraction of promising and diverse students to related technical areas at both undergraduate and graduate levels.

Each of the three thrusts will evolve as developments take place in the others. For example, the process development work will initially focus on measurements, models, and methods using existing AHSS. As candidate 3rd-generation AHSS become available in the second year, these methods will be applied to the new materials. GM has a critical role in all thrust areas to provide industrial input to help focus the research in each area. For the results of this research program to be realized, the results must be demonstrated as applicable to industry.

The 3 proposing organizations (and PI”s) and their principal roles are as follows:

• OSU: Lead partner on Process Development, supporting partner on Material Development, equal partner on Educational Program

• CSM: Lead partner on Material Development, supporting partner on Process Development, equal partner on Educational Program

• GM: Lead partner on digital image correlation and large-scale application testing, supporting partner on all three thrusts. (GM will provide $75,000 of cash support in addition to PI and other in-kind contributions.)

In addition to these principal parties, a broader group of participants has agreed to provide support for the project:

• Transportation Research Endowment Program (TREP): Will provide $75,000 in cash toward the OSU part of the project.

• Ohio Supercomputer Center (OSC): Will provide computational resources of $150,000 from this State of Ohio institution.

• AISI/AIST: Have provided a “FeMet” curriculum development grant of $25,000 to the first PI which will be incorporated into the Educational Program at OSU.

• Auto/Steel Partnership (A/SP) (members: Big 3 automakers plus 7 major steel companies): will provide existing AHSS alloys and will advise on choices of materials and forming operations

• Advanced Steel Processing and Products Research Center (ASPPRC): Will cooperate closely at CSM on joint student projects, and members will provide assistance with processsing selected materials.

There are many linkages among these participants, with common members. GM, for example, is represented by a PI and is a member of A/SP and SRC.

THRUST 1: PROCESS DEVELOPMENT (Springback and Formability)

Inability to predict formability and springback is the issue effectively limiting the use of new materials for sheet-formed applications. “Formability” refers to the materials ability to resist fracture under large-strain forming conditions. “Springback” is the elastically-driven shape change that occurs after forming material into a useful shape and removing it from the tooling.

Note: The inability to predict accurately the springback of standard steels is a major cost and factor of lead times even using well-established materials. One estimate for the cost to the U.S. automotive industry alone is $50 million per year [1]. With the introduction of 3rd-generation AHSS with complex microstrucures and new properties, the problem of predicting material response is magnified.

The springback objective is to provide knowledge and tools to enable reliable, accurate prediction of springback at the end of typical automotive forming operations. The following steps will be undertaken:

1. Measure strain hardening in complex, reverse paths (T/C testing) (OSU)

2. Devise advanced constitutive equations, implement in FE (OSU)

3. Measure springback; develop springback curves vs. R/t, tension (OSU, CSM)

4. Simulate springback; compare with measurements; validate (OSU)

During the first part of the project, these techniques will be applied to commercial alloys. As new alloy candidates become available (Thrust 2), they will be similarly tested.

The formability objective is to devise and validate a single methodology for measuring three sheet forming failure modes: 1) tensile localization (the dominant mechanism for traditional steel sheets), 2) shear fracture (observed for AHSS, not amenable to current analysis methods), and 3) edge-defect-initiated failure (more critical in AHSS). The following steps will be undertaken for 3rd-generation AHSS from Thrust 2[1]:

1. Devise a draw-bend formability test using ramped sheet tension (OSU, CSM)

2. Generate bending strain vs. tension strain failure maps (regions corresponding to failure type) (OSU)

3. Test strips of various widths to reveal the role of edge condition on failure, find critical width (OSU)

4. Begin to correlate the type of failure and failure limit with microstructure (CSM)

5. Develop digital image correlation (see Facilities and Resources) for imaging local strains near shear fractures (GM)

Both aspects of Thrust 1 rely on the draw-bend test architecture, developed by the first two PI’s [2-5]. The draw-bend test, Fig. 2, closely mimics the mechanics of deformation of sheet metal as it is drawn, stretched, bent, and straightened over a die radius entering a typical die cavity. It thus represents a wide range of sheet forming operations, but has the advantage of simplicity and the capability of careful control and measurement, particularly important for the sheet tension force. It provides a vital link to real forming practice, albeit in a simplified and more controlled setting. In-plane tests (e.g. tension test, Marciniak test) do not reproduce this condition, nor do stretch-dominated tests over large radii (FLD, LDH tests).

[pic]

Figure 2 – Draw-bend test a) special machine at OSU developed under NSF DMR 0139045, b) close-up of die area, CSM machine, and c) schematic of test and springback measures following it.

The original draw-bend test was devised for friction measurement under sheet forming conditions. The draw-bend springback test is conducted with a constant back force (sheet tension), Fb in Fig. 2a. It is the careful control of this force that gives significant advantages to the test over previous springback measurements. For the proposed draw-bend fracture implementation, the back force will be ramped up during the stroke in order to cause failure at some point during the test. Thus, for each R/t, a critical sheet tension causing failure will be determined.

Springback Testing, Simulation, and Related Constitutive Representation: The draw-bend springback test and associated simulations revealed that prediction of springback for press-formed parts is at least an order of magnitude more difficult and time consuming than prediction of sheet formability [6-9]. (Today, nearly every press-formed part in the automotive industry is simulated with consequently dramatically reduced lead times and tooling costs, but reliable springback prediction has been elusive.)

The problems identified with springback simulation include numerical effects and material behavior. Numerical tolerances, 1/10 of typical ones for forming simulation, are required. Up to 50 integration points through the sheet thickness (vs. 5-7 for forming). Mesh sizes may be required to be a fraction of those for forming [6-9]. Nonetheless, these phenomena are now well established and can be accommodated with sufficient computing power.

The shortcomings of material models measured for forming materials and used in commercial programs represent a more fundamental issue. An element of material in a sheet being drawn over a die radius undergoes a stress and strain reversal as it is first bent and then unbent. The entire sheet is under tension close to or exceeding the yield stress, and the reverse strains are large, the exact magnitude depending on R/t and sheet tension.

Deformation conditions with inherent strain reversals have little in common with proportional-path testing upon which most constitutive descriptions are based (tensile testing in various directions, plane-strain testing, balanced biaxial testing). The discovery that springback is sensitive to transient hardening like that shown in Fig. 3b motivated the development of a stabilized tension-compression test for sheet materials, Fig.3a [10-11]. Sequential tensile and compressive strains of up to 15% can be attained, sufficient for application to draw deformation over die radii.

[pic]

Figure 3 – Tension / compression test a) stabilizing fixtures [12,13], b) typical results for complex strain path (for 2524 aluminum alloy) [12].

Taking into account the behavior of Fig. 3b reduced the error of simulated springback by more than 50% [11]. A similar or larger effect on formability and fracture is expected for AHSS, which strain harden rapidly and are expected to exhibit extreme divergences in behavior under reverse straining conditions.

The reverse flow curves in Fig. 3b are typical of aluminum alloys. The transient response for steels is much more complex. The reverse hardening curve may exceed the forward one temporarily for AHSS, necessitating the development and use of advanced constitutive equations.

Complex hardening effects like those shown in Fig. 3b are not reproduced by isotropic hardening models, nor by simple forms of kinematic hardening. Based on Armstrong-Frederick-type hardening rules [14], Geng and Wagoner [11] introduced a variation of Chaboche’s formulation [15-17] based on a two-surface plasticity model which reproduces the strain-hardening behavior adequately. To model the permanent softening shown on reverse loading at larger prestrains, the bounding surface translates and expands according to a mixed hardening rule [18]. This model has been subsequently developed for more general behavior expected for a range of aluminum and AHSS alloys [12], and one- [19] and two-yield surface versions [20] have also appeared.

The complex hardening behavior of AHSS has not yet been measured. Doing so will be one of the first tasks; the results will guide subsequent constitutive developments. The constitutive model will be implemented for plane-stress thin-shell elements with an evolution of yield surface size and shape as described above, and using common anisotropic yield criteria: Hill'48 [21], Barlat's three-parameter yield function (Barlat Yld89) [22] and Barlat's Yld96 [23]. This material model will be implemented using a backward Euler algorithm for integration and a consistent tangent stiffness matrix will be derived, as described elsewhere [12].

A new phenomenon, time-dependent springback, was discovered by use of the draw-bend test [2,24]. It has been attributed to room-temperature creep driven by high residual stresses [24]. Initial tests showed that it occurred only for aluminum alloys; traditional steels (mild and HSLA grades) showed no effect even after up to 7 years following forming. However, recent unpublished research in preparation of this proposal shows that this time-dependent springback may be a complicating factor for AHSS, Fig. 4. AHSS draw-bend specimens will be measured over a period of months to detect whether this phenomenon occurs commonly in AHSS, and for which microstructures.

Figure 4 – First measurements of time-dependent springback in AHSS. Traditional steels show no effect for up to 7 years after forming. (R. H. Wagoner, unpublished research)

Formability Measurement: The formability of traditional ductile sheet alloys, e.g. steel, aluminum, and copper alloys, is well-represented by the Keeler-Goodwin diagram or Forming Limit Diagram (FLD). The FLD is a locus of points in strain space separating “safe” forming areas from areas that produce splitting failures. FLD’s represent failure by plastic localization or necking (similar to necking with a tensile specimen, but occurring under a biaxial state of stress).

Figure 5 - Shear failure and plastic localization: a) practical consequence for an AHSS automotive panel [Courtesy of the Auto/Steel Partnership] and b) shear failure of Mg AZ31B Alloy (left) vs. tensile localization of Al 6013 (right).

A second kind of forming failure has been observed for AHSS (but not for mild steels or HSLA steels): shear cracking without appreciable thinning [25], Fig. 5a. These failures occur over sharp radii and may propagate into interior regions of the part. FLD’s do not represent nor predict this mode of failure. Therefore, alternate tests and failure criterion representations are needed.

The difference between shear failure and plastic localization is illustrated in Fig. 5b, where tensile tests of a magnesium alloy (shear failure, left) and an automotive aluminum sheet alloy (necking failure, right) are compared.

Edge cracking is a third kind of failure. It has a minor impact for traditional steels but is important for AHSS. Note that in Fig. 5a some of the cracks originate at the edge of the formed part and propagate inward. Figure 6 shows that AHSS exhibit lower formability in the edge-sensitive, hole expansion test, in comparison to mild steels.

An ideal formability test for AHSS should be able to probe three conditions: 1) stretch-dominated failure (like a standard FLD), 2) bending-dominated shear failure, and 3) edge-condition dominated shear failure. The FLD will likely remain the mainstay for standard stretch-dominated failures.

The proposed “draw-bead fracture” test addresses all three of these conditions. The test will be run like the draw-bend springback test, with two changes: 1) the back force will be ramped up during the test (which would otherwise be constant) so that a failure occurs during the test, and 2) strips of varying width (the standard width for springback is 50mm) will be used to reveal at what width formability becomes significantly smaller. As envisioned, that critical width will be a measure of the sensitivity to edge condition.

The proposed procedure will detect both normal plastic localization/necking and shear fracture [26], Fig. 7. Therefore, a failure criterion constructed using these data will reflect both modes. Furthermore, the results will be closely applicable to typical sheet forming practice because of the similitude between deformation induced in a draw-bend test and during drawing of a sheet over a die radius.

The ratio of R/t determines the bending strain in the sheet. For each R/t, a critical backforce will be measured. The locus of critical R/t vs. Fb values will represent a new kind of forming limit diagram. Its variation with strip width will give information about center-sheet cracking vs. edge-dominated cracking. By recording the type of failure for each point, the relative sensitivity of a given AHSS will be discerned.

In later stages of the work, the fracture results will be correlated with microstructural features. Digital image correlation [27,28], as developed at GM, will be implemented as a means to observe strain localization during the draw-bend fracture test. This test has been used successfully to view strain gradients in 2-D tensile testing, and will enable quantifying strain in the more complex bending tests. (See Facilities and Resources.)

Materials: The initial list of existing materials to be evaluated, Table 1, was constructed with input from the Auto/Steel Partnership to match the most-considered candidates for automotive applications. A continuing selection of materials will be chosen based on availability and continuing assessments by the broad project team.

In addition to testing off-the-shelf material, at least one alloy will be alternately processed to obtain varying fractions of microstructures. For example, a D-P 400/600 will be reprocessed to obtain varying fractions of martensite and/or bainite, different hard particle sizes, different levels of "homogeneity" (i.e. distribution of the particles) while maintaining the ultimate tensile stress close to 600 MPa. Such a study will reveal the importance of microstructure changes on springback and formability at a given strength.

Table 1 - Typical mechanical properties for candidate alloys

|Alloy [Yield/UTS(MPa)] |Elongation |Type |

|DDQ (170/300) |44% |Baseline material, drawing-quality |

|HSLA 350/450 |30% |Baseline high-strength alloy |

|B-H 250350 |39% |Bake-hardenable |

|B-H 300/400 |36% |Bake-hardenable |

|D-R 250/400 |41% |Dent-resistant |

|D-P 300/500 |28% |Dual-phase |

|D-P 400/600 |24% |Dual-phase |

|TRIP 350/600 |32% |Tranformation Induced Plasticity |

|TRIP 600/800 |16% |Transformation Induced Plasticity |

|TWIP (450/1000) [29] |52% |Twinning Induced Plasticity |

|Gen 3 Alloys (TBD) |? |? |

Schedule: The plan for the Process Development thrust is as follows:

Year 1 • Measurement of plastic anisotropy using oriented tensile tests (existing AHSS)

• Measurement of complex hardening using tension/compression tests

• Measurement of draw-bend springback, time-dependent springback

• Development of the draw-bend fracture test

Year 2 • Development of advanced constitutive equations with complex hardening and anisotropy

• Fitting of anisotropy and complex hardening unified constitutive equations

• Draw-bend fracture testing of 3rd-generation AHSS alloys

• Begin T/C and springback testing for candidate 3rd -generation alloys

• Develop digital imaging correlation techniques for curved surfaces (GM)

Year 3 • Implementation of constitutive equations for FE usage

• Simulation of springback, comparison with simulations

• Digital imaging correlation of shear fractures

• Formulation of failure criterion based on draw-bend fracture results

THRUST 2: MATERIAL DEVELOPMENT (Microstructure Design)

AHSS microstructures of interest will be identified based on theoretical predictions of the deformation behavior of multi-phase sheet steels. The background for this Thrust 2 is summarized in a recent publication [1].

Processing and manufacturing methodologies for steels with strength/ductility combinations greater than available in the first generation of AHSS, but without the full cost of the stabilizing elements required for the austenitic second generation AHSS steels, will require unique alloy/microstructure/processing combinations. Several microstructural features must be simultaneously and independently controlled. These include the number of independent constituents (e.g. phases), and for each constituent the volume fraction, size, distribution, mechanical stability against strain, and mechanical properties (e.g. strength and strain hardening behavior).

Potential strength/ductility behaviors for new AHSS can be predicted with an appropriate deformation model that incorporates contributions of the individual microstructural constituents [1, 2]. Fig. 8 compares predictions of the effects of systematic microstructural variations, obtained by increasing the martensite volume fraction (up to 70 pct), in two hypothetical two-component composite materials of ferrite+martensite and austenite+martensite [1]. For these calculations, specific properties for each phase were assumed based on literature. Predictions were based on the application of a composite model for two ductile constituents [1]. Figure 8 suggests that the 3rd -generation AHSS will consist of complex microstructural combinations, with significant use of both high strength phases (e.g. martensite in this example) and highly strain hardening phases (e.g. austenite). Materials based on ferritic microstructures do not possess the required strain hardening capacity.

|[pic] |Figure 8: A comparison of predicted strength/ductility |

| |combinations for hypothetical composites of |

| |ferrite+martensite and stable austenite+martensite with |

| |property bands identified by AISI [3]. The predictions |

| |were based on a theoretical composite model with |

| |specific assumed stress strain curve shapes for each |

| |constituent as defined by UTS and uniform elongation, |

| |eu, values. The observed strength changes represent |

| |increasing volume fractions of martensite [1]. |

Thrust 2 has three primary research components.

1. Following the procedures used to develop the predictions in Fig. 8, ideal microstructures that lead to potential strength/ductility combinations characteristic of the third generation AHSS will be systematically identified based on theoretical predictions [1].

2. Novel laboratory processing methods will be employed to produce materials with controlled variations in phase volume fractions, distributions, and properties.

3. Experimental measurements of the mechanical properties, including formability, of the experimental steels will be obtained and compared to predictions based on the multi-phase models and used to refine the models, including those for springback and fracture developed in Thrust 1.

The results of this study will be incorporated into assessments of potential paths to process novel materials on a larger scale. Dta for the models will be obtained from the literature and direct measurements.

Assessment of Deformation Models for Multi-phase Steels: AHSS have complex multi-phase microstructures that consist of various combinations and distributions of ferrite, bainite, martensite, austenite, and carbide/nitride precipitates. If multiple ductile phases are load-bearing, then composite models based on various mixture rules (such as for an iso-strain composite) may be employed as shown in Fig. 8. Such models have been used successfully to describe strength/ductility combinations in dual phase [4] and metastable austenitic stainless steels [5]. If the load-bearing capacity of the second phase can be ignored, as is the case for a closely-spaced distribution of fine carbides in ferrite, then contributions to strength can be modeled based on conventional precipitation hardening models or with models that consider the effects of strain gradients between phases on dislocation accumulation and strength [6-8]. For each model considered, an expression for the stress-strain behavior of each individual constituent is required along with an expression that describes their interactions.

AHSS microstructural variables to be considered include, among others, grain size, solid solution alloy additions that affect austenite strain hardening behavior and stability with strain, precipitate strengthening, prior cold work, and extent of martensite tempering. Sets of predictive plots, similar to those shown in Fig. 8, will be developed and used to assess the significance of systematic microstructural changes. To illustrate the applicability of this approach to predict property variations, Figs. 9 and 10 compare the effects of modifying austenite properties in an austenite+martensite composite. In Fig. 9 austenite is assumed stable and in Fig. 10 the effects of austenite transformation to martensite with strain are illustrated.

|[pic] |Figure 9: The effects of variations in stable austenite |

| |mechanical properties on predicted strength/ductility |

| |combinations shown in Fig. 8. The properties of martensite |

| |were maintained constant (UTS = 2000 MPa; true uniform strain,|

| |εu = 0.08) and the austenite tensile strength was increased |

| |from 640 MPa, the value used for the plot in Fig. 8, to 850 |

| |MPa with a corresponding decrease in ductility [1]. |

In Fig. 9, data are shown for composites with two austenite strengths. The volume fraction of martensite was varied from 0 to 0.7 and the martensite properties were assumed constant. An increase in austenite strength from 640 MPa to 850 MPa, with a corresponding decrease in the true uniform strain from 0.6 to 0.4, is shown to lead to an increase in ductility at a given strength level. The observation that increasing the strength of one phase leads to a ductility increase is counterintuitive; it illustrates the importance of phase interactions on the overall properties of multi-phase steels.

The importance of controlling austenite stability with strain is illustrated for a series of hypothetical ferrite+austenite composites in Fig. 10. The austenite volume fraction is systematically increased and the effect of strain on austenite transformation to martensite is assumed to follow one of the four hypothetical stability curves in Fig. 10a. The dramatic effect on strength-ductility combinations is illustrated in Fig. 10b where predictions based on Mileiko’s model [2] are represented similar to the results in Fig. 8.

These predictions illustrate that excellent combinations of strength and ductility are only obtained for materials with high volume fractions of metastable austenite. Even though the predictions in Figs. 8-10 were based on a composite model with simplified assumptions [1, 2] and applied to hypothetical materials, the significance of the results is clear, and the methodology provides a clear understanding of the importance of the mechanical properties of the individual constituents in AHSS with complex microstructures. It is proposed that the methodologies identified here be applied to design of materials. Once desirable combinations of properties and microstructure are identified, then the methodologies required to manufacture the materials will be evaluated (next section). Input from partner GM will be critical here to assess material property requirements for future vehicles and provide guidance on desired material properties for processing as outlined below.

|[pic] |[pic] |

|Figure 10: The effects of austenite stability (a) on strength/ductility properties (b) in composites of ferrite and metastable |

|austenite. Calculations based on ferrite (UTS = 300 MPa, εu = 0.3, the true uniform strain), martensite (UTS = 2000 MPa, εu = |

|0.08), and austenite (UTS = 640 MPa, εu = 0.6). |

Production of Experimental Materials: Processing routes will be designed to produce experimental materials with systematic microstructural variations, and these materials will be used to evaluate the predictions from the models discussed above. It is anticipated that some of the microstructures of interest may involve small modifications to conventional routes while others may require alloy/microstructure combinations not readily available through conventional steel processing. Thus, to produce these types of materials, specialized novel laboratory experimental fabrication techniques, designed to produce materials for fundamental analyses, will be employed.

Available production methods to achieve materials with the desired microstructures will be assessed as part of this research program and those deemed applicable will be evaluated. One novel processing methodology that will be considered is roll bonding of multi-layered laminated composites. In a recent study, this technique has been used to produce steels with multiple layers, with thicknesses down to 10 (m. The effects of banding on microstructural evolution, mechanical properties, and hardenability were then evaluated [9, 10]. In the present concept, roll bonding may provide a simple means of preparing controlled composite steels.

The application of mechanical alloying and powder metallurgy techniques will be considered to produce materials with constituent compositions not readily available from conventionally solidified microstructures. Initial discussions have been held with organizations interested in partnering with us to produce the necessary materials.

Another processing route that has recently demonstrated significant promise is referred to as quenching and partitioning (Q&P). Q&P is a new process designed to produce controlled volume fractions of retained austenite in a martensitic matrix [11]. Tensile properties approaching those within the third generation AHSS band in Fig. 8 [12] have been produced. New alloys will be identified for Q&P processing to obtain phase distributions needed for 3rd-generation AHSS.

Several processing paths based on conventional materials will be considered. Metastable austenitic steels exhibit high strain hardening by martensite formation with strain [5]. These steels typically contain significant additions of austenite stabilizing elements, such as Mn, N, or Ni. One interesting possibility is to warm work austenitic stainless steels with very low manganese contents. In the annealed condition, these steels typically exhibit insufficient strength to achieve the strength/ductility combinations shown in Fig. 8 for the third generation AHSS. Application of warm work at a temperature where the austenite is stable will potentially lead to the required strength increase necessary to develop the desired strength/ductility combinations. Application of cold work to other multi-phase steels, e.g. duplex stainless steels, will also be considered.

Characterization of Material Properties, Summary: The experimental materials will be evaluated first in tension, primarily at room temperature and at quasi-static strain rates. The data will be analyzed to obtain both engineering and true stress versus strain curves and the resulting data will be correlated with the theoretical predictions of strength and flow behavior. Digital image correlation will be used to determine true stress/strain tensile properties beyond the point of necking. For each material, quantitative measurements of the microstructure will be obtained using standard light and electron microscopy techniques. (See Facilities and Resources.)

Schedule: Year 1: Develop property data sets for input into theoretical models (this will require an extensive literature review focused on the fundamental strengthening mechanisms of interest and may require mechanical testing of specific materials, designed to isolate properties of individual constituents). Initiate assessment of theoretical predictions. Agree on final experimental approach (including alloy design) with sponsor companies/agencies and initiate production of initial experimental materials. Initiate processing of potential alloys based on modifications to conventional steel processing technologies

Year 2: Refine model predictions and identify materials for study. Design and process materials based on alternate processing paths identified in year 1. Assess mechanical properties (Thrust 1) and microstructures of experimental materials and refine model predictions.

Year 3: Continue with processing and evaluation of experimental materials. Identify alternate material paths for consideration. Refine correlations between theoretical property predictions and measured properties. Document and communicate results.

BROADER IMPACT

Broader impacts in four principal areas are anticipated: 1) Benefits to Society, 2) Teaching, Training, Learning, 3) Broadened Participation, and 4) Dissemination of Results.

Benefits to Society: Advanced materials with high specific strengths offer many societal benefits in the form of better product performance, cost, safety (personal security), energy savings, emissions (especially greenhouse gases), reliability, and durability. In spite of these strong driving forces, adoption of these materials is limited by unknowns associated with design and manufacturing. These unknowns introduce uncertain tooling and tryout costs, and uncertain product lead times which effectively bar their widespread adoption and conferring of the potential benefits to mankind.

Two kinds of potential societal impact of replacing traditional HSLA steels with AHSS for automotive applications can be estimated quantitatively: 1) energy savings, and 2) environmental impact. These advances can further be divided into savings at the time of manufacture (production and processing) and throughout a vehicle’s service lifetime.

The potential energy and environmental savings of producing fewer tons of higher-strength steel may be estimated using the following figures provided by AISI [1]

• 30 million tons of flat rolled products for U.S. auto (3/4 of total production

• 10% reduction of steel tonnage by stronger, thinner sheets of AHSS (conservative estimate based on the ratio of yield and tensile strengths of ADSS vs. HSLA)

• 18 million BTU of energy consumed per ton of steel produced

• 1.7 tons of CO2 produced per ton of steel produced

• 0.5 of traditional steel auto parts replaced by AHSS.

The energy and CO2 production associated with sheet steel can be reduced as follows:

Energy saved (steel production) = 2.7 x 1013 BTU/year

CO2 saved (steel production) = 2.5x106 tons of CO2/year

The potential energy and environmental savings of using lighter automobiles may be estimated using the figures provided by the UltraLight Steel Auto Body project [2]:

• 3340 lb baseline sedan body reduced to 3035 lbs using advanced steels

• 305 lb mass savings x 1.5 (w/ secondary mass savings - powertrain, etc) = 458 lb

• 458 lb / 3340 = 13.7% weight reduction

• 13.7% weight reduction x 1/2 = 6.9% fuel economy improvement

• Baseline fuel economy = 26 mpg

One-year fuel savings, average sedan and fleet total (10 million vehicle produced/year):

12,205 miles per year / 26 mpg x 0.069 savings = 32.4 gallon/vehicle/year

32.4 gal/vehicle/year x 1x107 new vehicles = 324 x 106 gallons / year

Each gallon of gasoline contains 114,000 BTU [3] and burning each gallon of gasoline releases 19.6 pounds of CO2 [4], so service savings in the first year are thus:

324 x 106 gallons/year x 114,000 BTU/gallon = 37 x1012 BTU (1st year)

324 x 106 gal/year x 19.6 lb CO2/gallon / 2000 lb/ton = 3.2 x106 ton (1st year)

With an average vehicle lifetime of 10 years, the steady-state savings after ramping up of AHSS is.

Energy saved (service efficiency) = 3.7 x1014 BTU/year (steady state)

CO2 saved (service efficiency) =3.2 x107 tons CO2/year (steady state)

NOTE: The steady-state service energy savings achieved by introducing AHSS is equivalent to more than 30 400-megawatt power stations, enough to power 15 million typical American houses.

Teaching, Training, Learning – and – Broadened Participation

CSM and OSU are two of the few universities remaining in the United States that emphasize ferrous metallurgy in undergraduate and graduate curricula. CSM in particular emphasizes steel in all aspects of its curriculum.

The technical research program will be fully integrated into existing and new educational activities at the two institutions in three ways described below. The first two PI’s are uniquely positioned and committed to make this happen: Wagoner is Director of CAMMAC, the Center for Advanced Materials and Manufacturing Processes at OSU, and Matlock is Director of ASPPRC, the Advanced Steel Processing and Products Research Center at CSM.

Four specific activities are planned:

1) Graduate research associates will fulfill traditional roles in the project,

2) Undergraduate students will be hired at early stages in their careers to assist in the research and to help transition research into teaching material. Special attention to attracting under-represented groups for these positions will be made, along with a preference toward students who may qualify for graduate studies later on. At CSM, an undergraduate student is included in the NSF budget, and another will be cost-shared by ASPPRC resources. Students working on this project will report to the ASPPRC sponsors at their semi-annual review meetings held in Golden in March and September. At OSU, an undergraduate student will be cost-shared by the TREP program (see budget detail), and a second student will be supported in this area from a curriculum development grant from AISI and AIST (FeMet grant).

3) Educational modules based on technical results from this project will be developed as outgrowths of the project to build bridges: i) between mechanical and materials disciplines, ii) between graduate and undergraduate students, and iii) between traditional and nontraditional students. At CSM, such modules will be used in a required senior design course (MTGN 466 Design, Selection & Use of Materials), where faculty and centers in the Department sponsor senior design projects. For each year of this three year program, a senior design team will be assembled to work on one aspect of this overall design-based research program. At OSU, Wagoner will incorporate Level 2 modules into MSE 661: Ferrous Metallurgy, a senior course at OSU that attracts both undergraduate and graduate students from materials, mechanical, and industrial engineering programs. Funds to Wagoner from a 5-year curriculum development grant from AISI and the AIST Foundation that commenced in 2005 will leverage these developments.

4) GM will host summer interns, graduate and undergraduate, at its facilities during the project, both to augment mechanical property characterization with digital image correlation testing, and to assess materials with large-scale testing later in the project.

Both PIs are committed to advising graduate students from under-represented groups and mentoring undergraduate students through the transition to graduate education. 10 of the 44 graduate students advised by Wagoner to completion of their degrees are women who now work at a variety of places, including Honda of America, Idaho National Laboratory, British Petroleum R&D, and Daimler-Chrysler. Wagoner also welcomes incoming freshmen women engineers at The Ohio State University and attends local Society of Women Engineers conferences to help recruit female students into graduate education.

Dissemination of Results – The PIs are committed to wide dissemination of results in peer-reviewed journals, conference proceedings and lectures, graduate theses/dissertations, and undergraduate project reports. For the most-recent NSF grants to the first two PI’s, results were disseminated as follows:

DMR 0139045 (Wagoner) - 23 publications in print plus 10 in progress, 5 Ph.D. dissertations, 2 book chapters, 6 invited/international presentations, and numerous domestic or contributed presentations.

DMR 0303510 (Matlock) - 19 publications in print plus 10 in progress, 1 Ph.D. dissertation, 1 M.S. thesis, >16 invited / international presentations and numerous domestic or contributed presentations.

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hTÙ0J5?s have projects underway to develop and validate the draw-bend test to reveal shear fracture of existing AHSS. These projects will provide synergistic benefits to work proposed here: to apply these techniques to 3rd-generation AHSS.

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Figure 6 – Relative formability of AHSS vs traditional steel (mild) depending on edge condition. (Courtesy of Stuart P. Keeler and AISI)

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Figure 7 – Schematic of the proposed draw-bend fracture test with two modes of failure shown [25]

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