CROSS REFERENCE TO RELATED APPLICATION This application claims the benefit of U.S. Provisional Application 62/618,356 filed Jan. 17, 2018 which is fully incorporated herein by reference.
FIELD OF INVENTION This disclosure is related to alloys and methods of developing yield strength distributions during the formation of metal parts. Formation of metal parts through procedures such as stamping, especially for complex geometries, involves cold formability which requires ductility. The alloys herein provide improved yield strength distributions after formation which reduce cracking and other associated problems in metal part formation.
BACKGROUND Metal stamping involves a number of steps including successful forming of the stamping and achieving a targeted set of properties in the stamping. Successful forming of the stamping depends on the material properties including the global and local formability under a wide variety of stress states and strain rates. Sufficient cold formability is needed to produce the targeted geometry during the stamping operation after which a very limited material ductility remains in the stamping. This makes the stamping potentially susceptible to subsequent failure through various modes since the internal plasticity is not sufficient to develop an effective plastic zone in front of the crack tip to prevent crack propagation. Additionally, due to lack of remaining ductility, the metal stamping would also have a lack of toughness.
In metal stamping, the properties of the stamping are generally not specified as long as crack free stampings are produced. Instead, the properties of the sheet material utilized for stamping are stated. For conventional steels, properties in the stamped part are similar to that in the sheet material utilized since they undergo limited strain hardening during stamping operation and limited property changes.
As the development of steels has progressed, especially for autobody applications, it has been found that the increase in strength needed for lightweighting/gauge reduction results in the reduction in ductility/formability as shown by the “Banana plot” in FIG. 1. Thus, there exists a paradox of strength and ductility and as materials have become stronger, they have become less ductile/formable.
Accordingly, a need remains for the development of alloys and methods that would provide the ability to develop improved yield strength distributions during formation of metal parts, such that failure mechanisms such as cracking are eliminated or reduced, with an overall improvement in the number of successfully formed parts produced.
SUMMARY A method to develop yield strength distributions in a formed metal part comprising:
(a) supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Cr, Ni, Mn, Si, Cu, Al, or C, melting said alloy, cooling at a rate of <250 K/s, and solidifying to a thickness of 25.0 mm up to 500 mm;
(b) processing said alloy into sheet form with thickness from 0.5 to 10 mm wherein said sheet exhibits a yield strength of A1 (MPa), an ultimate tensile strength of B1 (MPa), a true ultimate tensile strength C1 (MPa), a total elongation D1(%);
(c) straining said sheet one or a plurality of times above said yield strength A1 at a strain rate of 100/s to 102/sec at an ambient temperature of 1° C. to 50° C. and forming a metal part having a distribution of yield strengths A2, A3, and A4, wherein:
A2=A1±100; (i)
A3>A1+100 and A3<A1+600; and (ii)
A4≥A1+600. (iii)
BRIEF DESCRIPTION OF THE DRAWINGS The detailed description below may be better understood with reference to the accompanying FIGS. which are provided for illustrative purposes and are not to be considered as limiting any aspect of this invention.
FIG. 1 World Auto Steel “Banana Plot”.
FIG. 2 Summary of yield strength distributions in strained parts.
FIG. 3 Stress—strain curve example for Alloy 8 showing the definition of 0.2%, 0.5% and 1.0% proof stresses as shown in enlarged image on the right.
FIG. 4 Summary of incremental tensile testing for Alloy 1 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 5 Summary of incremental tensile testing for Alloy 2 including; (a) the engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and (b) Yield strength and Fe % as a function of strain.
FIG. 6 Summary of incremental tensile testing for Alloy 3 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 7 Summary of incremental tensile testing for Alloy 4 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 8 Summary of incremental tensile testing for Alloy 5 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 9 Summary of incremental tensile testing for Alloy 6 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 10 Summary of incremental tensile testing for Alloy 7 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 11 Summary of incremental tensile testing for Alloy 8 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 12 Summary of incremental tensile testing for Alloy 9 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 13 Summary of incremental tensile testing for Alloy 10 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 14 Summary of incremental tensile testing for Alloy 11 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 15 Summary of incremental tensile testing for Alloy 12 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 16 Summary of incremental tensile testing for Alloy 13 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 17 Summary of incremental tensile testing for Alloy 14 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 18 Summary of incremental tensile testing for Alloy 15 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 19 Summary of incremental tensile testing for Alloy 16 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 20 Summary of incremental tensile testing for Alloy 17 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 21 Summary of incremental tensile testing for Alloy 18 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 22 Summary of incremental tensile testing for Alloy 19 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 23 Summary of incremental tensile testing for Alloy 20 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 24 Summary of incremental tensile testing for Alloy 21 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 25 Summary of incremental tensile testing for Alloy 22 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 26 Summary of incremental tensile testing for Alloy 23 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 27 Summary of incremental tensile testing for Alloy 24 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 28 Summary of incremental tensile testing for Alloy 25 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 29 Summary of incremental tensile testing for Alloy 26 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 30 Summary of incremental tensile testing for Alloy 27 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 31 Summary of incremental tensile testing for Alloy 28 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 32 Summary of incremental tensile testing for Alloy 29 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 33 Summary of incremental tensile testing for Alloy 30 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 34 Summary of incremental tensile testing for Alloy 31 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 35 Summary of incremental tensile testing for Alloy 32 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 36 Summary of incremental tensile testing for Alloy 33 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 37 Summary of incremental tensile testing for Alloy 34 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 38 Images of the microstructure in Alloy 7 sheet before deformation; a) SEM back-scattered image, b) TEM bright-field image, and c) HREM image of the nanoprecipitates.
FIG. 39 Images of the microstructure in Alloy 8 sheet before deformation; a) SEM back-scattered image, b) TEM bright-field image, and c) HREM image of the nanoprecipitates.
FIG. 40 Images of the microstructure in Alloy 7 sheet after deformation; a) SEM back-scattered image, and b) TEM bright-field image.
FIG. 41 Images of the microstructure in Alloy 8 sheet after deformation; a) SEM back-scattered image, and b) TEM bright-field image.
FIG. 42 Images of the Microconstituent 1 in the Alloy 8 sheet after deformation; a) TEM bright-field image, b) TEM dark-field image, c) TEM dark-field image of the ferrite grain at higher magnification, and d) HREM image of the nanoprecipitates.
FIG. 43 Images of the Microconstituent 2 in the Alloy 8 sheet after deformation; a) TEM bright-field image, b) TEM bright-field image of the deformed austenite grain at higher magnification showing dislocation cell structure, c) TEM image with highlighted nanoprecipitates by black circles, and d) HREM image of the nanoprecipitates.
FIG. 44 B-pillar surface with ˜20 mm grid pattern; a) Top section, b) Middle section 1, c) Middle section 2, and d) Bottom section.
FIG. 45 A histogram of Feritscope measurements across the surface of the B-pillar after 4 stamping hits. Note that the measurements showing baseline level of Fe % (i.e. <1%) are not shown on this plot.
FIG. 46 A histogram of Feritscope measurements across the surface of the B-pillar after 5 stamping hits. Note that the measurements showing baseline Fe % (i.e. <1%) are not shown on this plot.
FIG. 47 Tensile testing of specimens cut from the stamped B-pillar; a) A view of the B-pillar with marked specimen positions, and b) A view of the B-pillar after specimen cutting.
FIG. 48 Tensile properties of the Alloy 8 sheet measured by using ASTM E8 standard specimens and reduced size (i.e. 12.5 mm gauge) specimens.
FIG. 49 Stress—strain curve examples for specimens cut from the B-pillar with various levels of magnetic phases volume percent (Fe %).
FIG. 50 True stress—true strain curve examples for specimens cut from the B-pillar with various levels of magnetic phases volume percent (Fe %).
FIG. 51 Correlations of tensile properties with Feritscope; a) Strength characteristics vs corresponding measured Fe %, and b) Total elongation vs corresponding measured Fe %.
FIG. 52 Extrapolated correlations of tensile properties to the maximum Feritscope measurements of 31 Fe %; a) Strength characteristics, and b) Total elongation.
FIG. 53 Bright-field TEM micrographs of the microstructure in specimens cut from the stamped B-pillar before tensile testing and in the gauge of the tensile specimens after tensile testing with different levels of magnetic phases volume percent (Fe %); a) 4.6 Fe % sample before tensile deformation, b) 4.6 Fe % sample after tensile deformation, c) 13.9 Fe % sample before tensile deformation, d) 13.9 Fe % sample after tensile deformation, e) 24.5 Fe % sample before tensile deformation, and f) 24.5 Fe % sample after tensile deformation.
FIG. 54 Correlation of yield strength with magnetic phases volume percent (Fe %) for incremental tensile tested specimens and for tensile tested specimens cut from the B-pillar during destructive analysis.
FIG. 55 Summary of incremental tensile testing for Alloy 8 sheet with thickness of 0.5 mm including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 56 Summary of incremental tensile testing for Alloy 8 sheet with thickness of 1.3 mm including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 57 Summary of incremental tensile testing for Alloy 8 sheet with thickness of 3.0 mm including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 58 Summary of incremental tensile testing for Alloy 8 sheet with thickness of 7.1 mm including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength and Fe % as a function of strain.
FIG. 59 Summary of incremental tensile testing for TRIP 780 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength as a function of strain.
FIG. 60 Summary of incremental tensile testing for DP980 including; a) The engineering stress-strain curve, true stress—true strain curve, and incremental engineering stress-strain curves, and b) Yield strength as a function of strain.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Alloys herein can be initially produced in a sheet form by different methods of continuous casting including but not limited to belt casting, thin slab casting, and thick slab casting with achievement of advanced property combinations by subsequent post-processing. After processing into a sheet form as a hot band or cold rolled sheet, which may or may not be annealed, a preferred thickness of 0.5 mm to 10.0 mm is produced.
In FIG. 2 the achievement in alloy strengthening during stamping is illustrated. In Step 1 in FIG. 2, the starting condition is to supply a metal alloy. This metal alloy will comprise at least 70 atomic % iron. Preferably the level of iron is in the range of 70 atomic % iron to 85 atomic % iron. The metal alloy will contain at least four or more elements selected from Si, Mn, Cr, Ni, Cu, Al, or C. The alloy chemistry is melted, cooled at a rate of <250 K/s, and solidified to a thickness of 25 mm and up to 500 mm.
The casting step can preferably be done in a wide variety of processes including ingot casting, bloom casting, continuous casting, thin slab casting, thick slab casting, belt casting etc. Preferred methods would be continuous casting in sheet form by thin slab casting or thick slab casting. To produce alloys herein in a sheet form, the casting processes can vary widely depending on specific manufacturing routes and specific targeted goals. As an example, consider thick slab casting as one process route to produce sheet product. The alloy would be cast going through a water cooled mold typically in a thickness range of 150 to 350 mm in thickness and typically processed through a roughing mill hot roller into a transfer bar slab of 25 to 150 mm in thickness and through the finishing mill into a hot band with thickness of 1.5 to 10.0 mm. Another example would be to preferably process the cast material through a thin slab casting process. In this case, after casting typically forms 25 to 150 mm in thickness by going through a water cooled mold, the newly formed slab goes directly to hot rolling without cooling down and the strip is rolled into hot band coils with typical thickness from 1.5 to 5.0 mm in thickness. Note that bloom casting would be similar to the examples above but higher thickness might be cast typically from 200 to 500 mm thick and initial breaker steps would be needed to reduce initial cast thickness to allow it to go through a hot rolling roughing mill.
Step 2 in FIG. 2 corresponds to sheet product from alloys herein with preferred thickness from 0.5 to 10 mm. The processing of the cast material in Step 1 into sheet form can preferably be done by hot rolling, forming a hot band. Produced hot band may be further processed towards smaller gauges by cold rolling that can be applied at various reductions per pass, variable number of passes and in different mills including tandem mills, Z-mills, and reversing mills. Typically cold rolled thickness would be 0.5 to 10 mm thick. Preferably, the cold rolled material is annealed to restore the ductility lost from the cold rolling process either partially or completely.
Preferably, sheet material from the alloys herein have a yield strength of A1 (250 MPa to 750 MPa), a tensile strength of B1 (700 MPa to 1750 MPa), a true ultimate tensile strength of C1 (1100 MPa to 2300 MPa), and exhibits a total elongation D1 (10% to 80%). While engineering stress is determined as the applied load divided by the original cross-sectional area of the specimen gauge, true stress corresponds to the applied load divided by the actual cross-sectional area (the changing area with respect to time) of the specimen at that load. True stress is the stress determined by the instantaneous load acting on the instantaneous cross-sectional area. True ultimate tensile strength (C1) is related to ultimate tensile strength (B1) and can be calculated from the test data for each alloy herein using Eq.1. Engineering strain is determined as the change in length divided by the original length. Calculated true ultimate tensile strength values vary from 1165 to 2237 MPa:
True Ultimate Tensile Strength(C1)=Ultimate Tensile Strength*(1+Engineering Strain) (Eq.1)
True strain at fracture corresponding to total elongation of each specimen was calculated by Eq.2. True strain at fracture was found to vary from 15.7 to 58.1%.
True Strain at Fracture=In(1+Engineering Strain) (Eq.2)
Depending on alloy chemistry, the magnetic phase volume percent generally varies from 0.2 to 45.0 Fe % for hot band or cold rolled and annealed sheet. Such magnetic phase volume is then increased as discussed more fully below.
Straining of the alloy sheet above its yield strength, which may preferably occur via stamping of the sheet from said alloy with the indicated influence on yield strength occurring during the stamping operation, is shown by Step 3 in FIG. 2. The alloy is permanently (i.e. plastically) deformed during the stamping operation, preferably at strain rates of 100/s to 102/s which is reference to deformation when yield strength is exceeded. Metal stamping is the process of placing sheet metal at ambient temperature and without external heating in either blank or coil form into a stamping press where a tool and die surface forms the metal into a net shape. Ambient temperature may preferably be understood as a temperature range from 1° C. to 50° C., more preferably 1° C. to 40° C., and even more preferably 5° C. to 30° C. Note that during stamping, the blank as it is formed does experience internal heating from the stamping process which includes both frictional heating and deformation induced heating. The internal blank heat up during stamping is generally less than 150° C. and typically less than 100° C. This could be a single stage operation where every stroke of the press produces the desired form on the sheet metal part, or could occur through a series of stages, generally 2 to 7 but may occur in up to 25 stages, where each stage where the formed or partially formed metal part is deformed introduces a deformation that exceeds the yield strength of the material of previous step. Note that during each stage/press stroke, the localized deformation will vary by location so a multitude of different strains will be applied concurrently during the stamping operation and as noted, preferably at strain rates from 100/s to 102/s. Formability is the primary attribute of sheet metal material to undergo forming, in the plastic regime (i.e. forming at the point where yield strength is exceeded), which involves material straining during bending, stretching, and drawing etc. depending on stamping geometry.
The alloys herein undergoing what is illustrated in FIG. 2 may also preferably be characterized based upon the microstructure transformations when deformed above the yield strength. This is termed a Nanophase Refinement & Strengthening (NR&S) mechanism that preferably occurs with formation of new microstructure defined by two Microconstituents. Initial sheet microstructure is such that it contains areas with stable austenite meaning that it will not change into the ferrite phase during deformation and areas with relatively unstable austenite, meaning that it is available for transformation into ferrite upon plastic deformation. Upon deformation, the areas with relatively unstable austenite undergo transformation into ferrite particles with a nanoscale size from 20 nm to 750 nm (longest linear dimension) forming Microconstituent 1 along with the formation of nanoprecipitates in the range of 2 to 100 nm in size (longest linear dimension) and contributing to material strengthening due to structural refinement. As this ferrite phase forms, it continues to deform through a dislocation mechanism contributing to sheet ductility and formability.
Areas of the microstructure in the initial sheet from the alloys herein with relatively stable austenite retain the austenitic nature but deform through primarily dislocation mechanisms supporting material ductility and formability during stamping and forming Microconstituent 2 in the final microstructure after deformation. Microconstituent 2 itself contains two components which are micron sized stable austenite particles, typically 1.0 to 10.0 microns in size (longest linear dimension) and nanoprecipitates typically 2 to 100 nm in size (longest linear dimension). Nanoprecipitates in either Microconstituent 1 or 2 can be directly observed through TEM microscopy and are observed to exhibit a spherical, elliptical, or rectangular shape in the size range indicated. To further identify, selected area diffraction in the TEM on the precipitates can be done to show that they have different structures (i.e. not FCC austenite or BCC Ferrite) than the matrix phases (i.e. austenite which is FCC or alpha ferrite which is BCC). Accumulation of dislocations within micron-sized austenite grains results in dislocation cell block boundaries, and dislocation cell formation leading to material strengthening. Additionally, as noted, nanoprecipitates with a size from 2 to 100 nm are present in both Microconstituents 1 and 2 also contributing to material strengthening.
The resulting volume fraction of Microconstituent 1 and Microconstituent 2 in the localized areas of the stamping, i.e., the final formed part, depends on alloy chemistry, the level of straining at particular location, and the level of strain hardening which occurs during the single or multistage stamping operation. Note that the microstructure and resulting properties will change in the stamped part from the starting sheet/blank depending on the local level of straining. Typically, as low as 1 volume percent and as high as 85 volume percent of the alloy structure after stamping will exist as the ferrite containing Microconstituent 1 with the remaining regions representing Microconstituent 2. Thus, Microconstituent 1 can be in all individual volume percent values from 0.5 to 85.0 in 0.1% increments (i.e. 0.5%, 0.6%, 0.7%, . . . up to 85.0%) while Microconstituent 2 can be in volume percent values from 99.5 to 15 in 0.1% increments (i.e. 99.5%, 99.4%, 99.3% . . . down to 15.0%). The volume percent of nanoprecipitates which occur in both microconstituents is anticipated to be 0.1 to 10%. While the magnetic properties of these nanoprecipitates are difficult to individually measure, it is contemplated that they are non-magnetic.
As ferrite is magnetic (i.e. ferromagnetic), and austenite is non-magnetic (i.e. paramagnetic), the volume fraction of the magnetic phases present provides a convenient method to evaluate the relative presence of Microconstituent 1. The magnetic phases volume percent is abbreviated herein as Fe %, which should be understood as a reference to the presence of ferrite and any other components in the alloy that identifies a magnetic response such as alpha-martensite. Note that the alpha-ferrite and alpha-martensite have similar magnetic responses and cannot be distinguished separately by the Feritscope so both will be identified as ferrite. Magnetic phase volume percent herein is conveniently measured by a Feritscope. The Feritscope uses the magnetic induction method with a probe placed directly on the sheet sample and provides a direct reading of the total magnetic phases volume percent (Fe %). After cold deformation, the volume fraction of Microconstituent 1 is estimated using the measured Fe % value which can include alpha-ferrite and/or alpha-martensite. Microconstituent 2 which is nonmagnetic and cannot be measured by the Feritscope, would then be considered the remaining constituent.
While the multiple mechanistic components of the NR&S mechanism described above support deformation of the sheet during its forming into targeted shape, sheet material from alloys herein undergoes a substantial strain hardening/strengthening which results in the presence of distributions (i), (ii), and (iii) in the formed parts provided in FIG. 2. Depending on alloy chemistry, the level of straining at particular location, and local stress state during stamping operation occurring without external application of heat, results in different levels of strengthening leading to three distributions of yield strength characteristics in the formed part as show in Step 4 in FIG. 2: (i) yield strength A2 (MPa) whereby A2=A1±100; (ii) yield strength A3 (MPa) whereby A3>A1+100 and A3<A1+600; and (iii) yield strength A4 (MPa) whereby A4≥A1+600 and A4≤C1. Distribution (iii) represents a maximum level of strengthening in the formed part with yield strength A4 in the range from 850 to 2300 MPa. In addition, it should be noted that preferably, yield strength distributions (i), (ii) and (iii) are the only yield strengths that are present in the formed part, except for reduced yield strengths that are attributed to defects in the parts that can occur due to casting and subsequent processing. Such defects therefore can include, e.g., internal cavities (voids), slag from casting, microcracks, or inclusions.
Forming of the alloys herein can be done by various methods including but not limited to forming in single and/or progressive dies and with one stage or multiple stages up to 25 towards targeted final form using a combination of techniques, without external heating, including but not limited to stamping, roll forming, metal drawing, and hydroforming. In connection with such procedures the deformation that exceeds the yield strength may include hole expansion, hole extrusion drawing, bending and/or stretching. Common to all of these processing techniques is the introduction of a one or a plurality of deformations (introduction of strain) such that yield strength is exceeded with the result that all of the above referenced distribution of yield strengths are achieved in the formed part. The final formed part applications include but are not limited to automotive industry (a vehicular frame, vehicular chassis, or vehicular panel), and/or railroad industry (a storage tank, freight car, or railway tank car).
Main Body Alloys The chemical composition of the alloys herein is shown in Table 1 which provides the preferred atomic ratios utilized.
TABLE 1
Chemical Composition Of Alloys (Atomic %)
Alloy Fe Cr Ni Mn Al Si Cu C
Alloy 1 76.17 8.64 0.90 11.77 — — 1.68 0.84
Alloy 2 78.17 1.85 11.42 — — 3.94 2.68 1.94
Alloy 3 76.55 0.78 0.72 14.43 — 3.42 0.42 3.68
Alloy 4 80.90 3.69 4.97 5.57 — 2.98 0.37 1.52
Alloy 5 75.67 2.63 3.40 11.03 — 5.13 1.35 0.79
Alloy 6 71.68 6.25 10.45 0.62 2.63 5.22 1.64 1.51
Alloy 7 75.75 2.63 1.19 13.86 — 5.13 0.65 0.79
Alloy 8 74.75 2.63 1.19 14.86 — 5.13 0.65 0.79
Alloy 9 74.59 2.61 — 15.17 — 3.59 1.86 2.18
Alloy 10 73.75 2.63 1.19 15.86 — 5.13 0.65 0.79
Alloy 11 80.93 — 2.68 12.04 — 0.79 0.89 2.67
Alloy 12 73.95 2.60 1.18 14.7 1.08 5.07 0.64 0.78
Alloy 13 80.89 0.43 0.42 14.82 — 2.03 1.41 —
Alloy 14 77.46 — — 15.42 — 3.78 1.73 1.61
Alloy 15 81.51 2.45 3.78 11.79 — — — 0.47
Alloy 16 79.02 — 2.95 10.88 — 5.18 1.97 —
Alloy 17 75.55 1.67 1.63 14.92 — 6.23 — —
Alloy 18 76.62 2.63 7.85 4.42 — 5.13 2.61 0.74
Alloy 19 77.17 1.85 12.42 — — 3.94 2.68 1.94
Alloy 20 70.92 2.5 1.13 14.1 5.11 4.87 0.62 0.75
Alloy 21 71.96 2.53 1.15 14.31 3.72 4.94 0.63 0.76
Alloy 22 72.77 2.56 1.16 14.47 2.65 4.99 0.63 0.77
Alloy 23 77.35 — 2.56 11.51 4.42 0.76 0.85 2.55
Alloy 24 79.85 — 1.34 12.04 2.42 0.79 0.89 2.67
Alloy 25 78.86 0.29 0.78 14.41 2.68 0.87 0.96 1.15
Alloy 26 74.05 2.63 — 12.04 4.71 5.13 0.65 0.79
Alloy 27 76.83 — 3.47 13.67 0.42 2.78 2.45 0.38
Alloy 28 75.21 2.63 — 12.04 4.34 5.13 0.65 —
Alloy 29 73.63 2.63 — 12.04 4.34 5.13 0.65 1.58
Alloy 30 75.57 1.32 — 13.57 4.00 4.43 0.32 0.79
Alloy 31 77.00 — — 13.13 4.00 4.43 0.65 0.79
Alloy 32 73.52 3.26 — 12.14 4.61 4.07 0.29 2.11
Alloy 33 75.69 4.59 0.37 14.16 3.20 — 0.48 1.51
Alloy 34 70.45 1.49 0.55 16.85 0.87 6.22 1.85 1.72
With regards to the above, and as can be further seen from Table 1, preferably, when Fe is present at a level of greater than 70 at. %, and one then selects the four or more elements from the indicated seven (7) elements, or selects five or more elements, or selects six or more elements or selects all seven elements to provide a formulation of elements that totals 100 atomic percent. The preferred levels of the elements, if selected, may fall in the following ranges (at. %): Cr (0.2 to 8.7), Ni (0.3 to 12.5), Mn (0.6 to 16.9), A1 (0.4 to 5.2), Si (0.7 to 6.3), Cu (0.2 to 2.7), and C (0.3 to 3.7). Accordingly, it can be appreciated that if four (4) elements are selected, two of the six elements are not selected and may be excluded. If five (5) elements are selected, one of the elements of the six can be excluded. Moreover, a particularly preferred level of Fe is in the range of 70.0 to 85.0 at. %. The level of impurities of other elements is in the range of 0 to 5000 ppm. Accordingly, if there is 5000 ppm of an element other than the selected elements identified, the level of such selected elements may then in combination be present at a lower level to account for the 5000 ppm impurity, such that the total of all elements present (selected elements and impurities) is 100 atomic percent.
The alloys herein were processed into a laboratory sheet by processing of laboratory slabs. Laboratory alloy processing is developed to mimic closely the commercial sheet production by continuous casting and include hot rolling and cold rolling. Annealing might be applied depending on targeted properties. Produced sheet can be used in hot rolled (hot band), cold rolled, annealed, or partially annealed states.
Laboratory Slab Casting Alloys were weighed out into 3,000 to 3,400 gram charges according to the atomic ratios in Table 1 using commercially available ferroadditive powders and a base steel feedstock with known chemistry. Impurities can be present at various levels depending on the feedstock used. Impurity elements would commonly include the following elements; Co, N, P, Ti, Mo, W, Ga, Ge, Sb, Nb, Zr, O, Sn, Ca, B and S which if present would be in the range from 0 to 5000 ppm (parts per million) (0 to 0.5 wt %) at the expense of the desired elements noted above. Preferably, the level of impurities is controlled to fall in the range of 0 to 3000 ppm (0.3 wt %).
Charges were loaded into a zirconia coated silica crucible which was placed into an Indutherm VTC800V vacuum tilt casting machine. The machine then evacuated the casting and melting chambers and flushed with argon to atmospheric pressure twice prior to casting to prevent oxidation of the melt. The melt was heated with a 14 kHz RF induction coil until fully molten, approximately from 5 to 7 minutes depending on the alloy composition and charge mass. After the last solids were observed to melt it was allowed to heat for an additional 30 to 45 seconds to provide superheat and ensure melt homogeneity. The casting machine then evacuated the chamber and tilted the crucible and poured the melt into a water cooled copper die. The melt was allowed to cool under vacuum for 200 seconds before the chamber was filled with argon to atmospheric pressure.
Physical Properties of Cast Alloys A sample of between 50 and 150 mg from each alloy herein was taken in the as-cast condition. This sample was heated to an initial ramp temperature between 900° C. and 1300° C. depending on alloy chemistry, at a rate of 40° C./min. Temperature was then increased at 10° C./min to a max temperature between 1425° C. and 1510° C. depending on alloy chemistry. Once this maximum temperature was achieved, the sample was cooled at a rate of 10° C./min back to the initial ramp temperature before being reheated at 10° C./min to the maximum temperature. Differential Scanning Calorimetry (DSC) measurements were taken using a Netzsch Pegasus 404 DSC through all four stages of the experiment, and this data was used to determine the solidus and liquidus temperatures of each alloy, which are in a range from 1294 to 1498° C. (Table 2). Depending on the alloys chemistry, liquidus-solidus gap varies from 26 to 138° C. Thermal analysis provides information on maximum temperature for the following hot rolling processes that varies depending on alloy chemistry.
TABLE 2
Thermal Analysis Of Alloys
Alloy Solidus (°C.) Liquidus (°C.) Melting Gap (°C.)
Alloy 1 1406 1488 82
Alloy 2 1460 1489 29
Alloy 3 1294 1432 138
Alloy 4 1430 1481 51
Alloy 5 1419 1455 36
Alloy 6 1350 1441 91
Alloy 7 1390 1448 58
Alloy 8 1395 1443 48
Alloy 9 1358 1445 87
Alloy 10 1385 1443 58
Alloy 11 1456 1491 35
Alloy 12 1377 1457 80
Alloy 13 1464 1490 26
Alloy 14 1398 1452 54
Alloy 15 1471 1498 27
Alloy 16 1419 1458 39
Alloy 17 1392 1450 58
Alloy 18 1421 1461 40
Alloy 19 1416 1464 48
Alloy 20 1346 1456 110
Alloy 21 1361 1457 95
Alloy 22 1376 1448 72
Alloy 23 1423 1472 49
Alloy 24 1430 1486 56
Alloy 25 1439 1482 43
Alloy 26 1347 1466 119
Alloy 27 1426 1464 38
Alloy 28 1385 1470 85
Alloy 29 1342 1459 117
Alloy 30 1397 1474 77
Alloy 31 1389 1479 90
Alloy 32 1377 1454 77
Alloy 33 1420 1478 58
Alloy 34 1400 1452 52
The density of the alloys herein was measured on samples from hot rolled material using the Archimedes method in a specially constructed balance allowing weighing in both air and distilled water. The density of each alloy is tabulated in Table 3 and was found to be in the range from 7.48 to 8.01 g/cm3. The accuracy of this technique is ±0.01 g/cm3.
TABLE 3
Density Of Alloys
Alloy Density (g/cm3)
Alloy 1 7.89
Alloy 2 7.92
Alloy 3 7.77
Alloy 4 7.90
Alloy 5 7.80
Alloy 6 7.69
Alloy 7 7.78
Alloy 8 7.77
Alloy 9 7.78
Alloy 10 7.77
Alloy 11 7.93
Alloy 12 7.72
Alloy 13 7.94
Alloy 14 7.80
Alloy 15 8.01
Alloy 16 7.83
Alloy 17 7.77
Alloy 18 7.86
Alloy 19 7.93
Alloy 20 7.48
Alloy 21 7.56
Alloy 22 7.63
Alloy 23 7.69
Alloy 24 7.80
Alloy 25 7.79
Alloy 26 7.49
Alloy 27 7.90
Alloy 28 7.51
Alloy 29 7.50
Alloy 30 7.57
Alloy 31 7.59
Alloy 32 7.50
Alloy 33 7.73
Alloy 34 7.82
Laboratory Processing into Sheet Through Hot Rolling, Cold Rolling, and Annealing The alloys herein were preferably processed into a laboratory hot band by hot rolling of laboratory slabs at high temperatures. Laboratory alloy processing is developed to simulate the hot band production from slabs produced by continuous casting. Industrial hot rolling is performed by heating a slab in a tunnel furnace to a target temperature, then passing it through either a reversing mill or a multi-stand mill or a combination of both to reach the target gauge. During rolling on either mill type, the temperature of the slab is steadily decreasing due to heat loss to the air and to the work rolls so the final hot band is formed at a reduced temperature. This is simulated in the laboratory by heating in a tunnel furnace to between 1100° C. and 1250° C., then hot rolling. The laboratory mill is slower than industrial mills causing greater loss of heat during each hot rolling pass so the slab is reheated for 4 minutes between passes to reduce the drop in temperature, the final temperature at target gauge when exiting the laboratory mill commonly is in the range from 800° C. to 1000° C., depending on furnace temperature and final thickness.
Prior to hot rolling, laboratory slabs were preheated in a Lucifer EHS3GT-B18 furnace. The furnace set point varies between 1100° C. to 1250° C., depending on alloy melting point and point in the hot rolling process, with the initial temperatures set higher to facilitate higher reductions, and later temperatures set lower to minimize surface oxidation on the hot band. The slabs were allowed to soak for 40 minutes prior to hot rolling to ensure they reach the target temperature and then pushed out of the tunnel furnace into a Fenn Model 061 2 high rolling mill. The 50 mm casts were hot rolled for 5 to 10 passes though the mill before being allowed to air cool. Final thickness ranges after hot rolling are preferably from 1.8 mm to 4.0 mm with variable reduction per pass ranging from 20% to 50%.
Hot band material was media blasted prior to cold rolling to remove surface oxides which could become embedded during the rolling process. The resultant cleaned sheet material was rolled using a Fenn Model 061 2 high rolling mill down to 1.2 mm thickness. Reductions before annealing ranged from 10% to 40%.
Once the final gauge thickness of 1.2 mm was reached, tensile samples were cut from the laboratory sheet by wire-EDM. The samples were annealed under conditions intended to simulate the thermal exposure expected during an industrial continuous annealing process representing final treatment of sheet material in Step 2 in FIG. 2. Samples were wrapped in stainless steel foil to prevent oxidation and loaded into a preheated furnace at 850° C. Samples were left in the furnace for 10 minutes while the furnace purged with argon before being removed and allowed to air cool.
Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control at a constant displacement rate of 0.036 mm/s. Tensile properties of 1.2 mm thick sheet from alloys herein after annealing at 850° C. for 10 minutes are listed in Table 4. The ultimate tensile strength values of the annealed sheet from alloys herein is in a range from 717 to 1683 MPa with total elongation recorded in the range from 17.1 to 78.9%. The 0.2% proof stress varies from 273 to 652 MPa, 0.5% proof stress varies from 295 to 704 MPa, and 1.0% proof stress varies from 310 to 831 MPa. True ultimate tensile strength calculated from the data for each alloy herein, which varies from 1188 to 2237 MPa with true strain at fracture from 15.7 to 58.1%.
As the exact point of yielding is difficult to determine, a range of proof tests were employed at 0.2%, 0.5% and 1.0% proof stresses. That is, the exact point where the deformation changes from elastic to plastic is complicated by the unique deformation mechanisms of the alloys herein, resulting in a curvature of the initial portion of the stress strain curve. The 0.2%, 0.5%, and 1.0% represent offset strains whereby at these strain levels, a parallel line is drawn to the stress strain curve and the resulting points of intersection is defined at the proof stress at the identified offsets respectively. At the 0.5% proof stress, more consistent and representative values are obtained so that the yield strength herein (A1, A2, A3, and A4) will be defined at the 0.5% proof stress. In FIG. 3, a Stress—strain curve example is provided showing the definition of 0.2%, 0.5% and 1.0% proof stresses. As can be seen from Table 4 below, the 0.5% proof stress, or yield strength of the sheet (A1), ranges from 295 MPa to 704 MPa. Therefore, it is contemplated herein that the alloy sheet made from the alloys herein will have a yield strength in the range of 250 MPa to 750 MPa.
TABLE 4
Tensile Properties of Final Sheet After Annealing at 850° C. For 10 min
True
Ultimate 0.5% Ultimate
Total Tensile 0.2% Proof 1.0% True Tensile
Elongation Strength Proof Stress Proof Strain At Strength
(%) (MPa) Stress (MPa) Stress Fracture (MPa)
Alloy D1 B1 (MPa) A1 (MPa) (%) C1
Alloy 1 50.1 1175 483 524 552 40.6 1714
50.9 1161 472 514 544 41.1 1692
50.8 1190 471 517 548 41.1 1731
Alloy 2 36.6 1659 292 341 405 31.2 2237
31.0 1683 317 357 392 26.9 2202
34.7 1683 292 351 439 29.7 2159
37.3 1655 286 339 418 31.6 2194
Alloy 3 60.3 1134 499 516 534 47.2 1767
58.2 1141 500 518 536 45.9 1775
60.4 1139 500 517 535 47.3 1778
64.2 1138 490 508 526 49.6 1814
Alloy 4 31.5 1497 416 441 466 27.4 1953
35.6 1542 419 444 472 30.4 2042
35.3 1504 423 447 477 30.2 2026
42.3 1539 420 447 479 35.3 2182
Alloy 5 56.8 1165 386 422 448 45.0 1804
67.5 1129 440 471 490 51.6 1828
58.5 1136 396 425 449 46.1 1733
62.2 1137 389 421 447 48.3 1804
Alloy 6 18.2 1532 474 577 793 16.6 1726
18.7 1544 475 584 804 17.1 1742
17.1 1539 488 603 831 15.7 1714
19.0 1540 468 561 773 17.3 1743
Alloy 7 55.7 1267 469 495 523 44.3 1873
52.0 1242 456 485 513 41.9 1819
56.0 1248 470 499 525 44.5 1874
57.7 1277 464 489 515 45.6 1887
Alloy 8 65.4 1162 491 513 537 50.3 1841
59.4 1179 469 496 522 46.6 1812
61.8 1193 477 502 528 48.2 1836
62.6 1172 531 556 578 48.6 1806
Alloy 9 64.7 993 484 504 522 49.9 1574
66.1 997 491 512 530 50.7 1592
66.2 994 481 503 520 50.8 1593
66.3 994 491 510 526 50.9 1587
Alloy 10 63.9 1102 463 489 514 49.4 1772
63.5 1118 465 492 518 49.2 1792
65.3 1127 478 503 528 50.2 1784
70.8 1108 475 503 527 53.5 1816
62.6 1112 473 498 523 48.6 1765
Alloy 11 66.4 892 326 337 351 50.9 1457
61.6 876 319 323 336 48.0 1398
64.2 889 322 335 348 49.6 1437
67.5 886 321 327 339 51.5 1447
Alloy 12 60.4 1129 423 460 489 47.2 1748
65.3 1136 440 470 497 50.2 1807
63.0 1144 421 458 487 48.8 1776
63.8 1129 427 462 490 49.3 1785
Alloy 13 49.5 987 388 432 459 40.2 1403
48.7 988 381 419 446 39.7 1392
49.0 991 358 406 442 39.9 1405
44.2 999 377 414 441 36.6 1367
Alloy 14 72.9 1035 413 446 473 54.7 1704
70.2 1016 407 440 466 53.1 1653
73.7 1056 429 460 485 55.2 1754
74.3 1032 406 441 468 55.5 1729
Alloy 15 42.5 1170 273 319 354 35.3 1605
40.5 1164 295 327 358 34.0 1551
43.3 1164 283 321 354 35.9 1563
41.9 1175 296 329 360 34.9 1574
Alloy 16 39.5 1196 366 394 407 33.2 1586
39.6 1196 377 401 413 33.3 1579
38.4 1213 377 405 421 32.5 1601
39.3 1187 355 386 400 33.1 1573
Alloy 17 51.1 1070 402 438 478 41.3 1547
51.8 1073 405 447 485 41.7 1565
54.3 1060 381 423 466 43.3 1552
57.9 1067 395 435 476 45.6 1593
Alloy 18 53.4 1111 320 323 329 42.7 1631
49.7 1110 314 312 316 40.3 1607
54.3 1102 300 301 310 43.4 1602
50.7 1115 334 333 335 41.0 1591
Alloy 19 43.0 1471 315 328 371 35.7 2090
48.4 1449 314 331 376 39.5 2107
45.0 1505 317 331 372 37.1 2116
41.7 1478 316 329 370 34.9 2086
Alloy 20 78.6 887 455 476 499 58.0 1514
78.9 888 459 481 504 58.1 1513
78.5 880 455 481 502 58.0 1500
77.7 890 467 490 512 57.5 1512
Alloy 21 70.5 1016 465 502 528 53.3 1649
71.2 1005 465 502 528 53.8 1650
69.1 1001 459 494 519 52.5 1621
Alloy 22 66.3 1071 464 499 524 50.8 1713
66.8 1072 463 498 524 51.2 1710
64.1 1104 466 503 531 49.5 1722
65.7 1093 459 497 525 50.4 1722
Alloy 23 76.4 762 355 359 370 56.7 1284
73.1 756 350 352 365 54.8 1267
76.4 761 356 359 371 56.7 1297
72.0 755 352 354 367 54.2 1260
Alloy 24 67.4 838 339 343 353 51.5 1371
65.3 825 333 338 349 50.3 1342
62.3 830 336 342 352 48.5 1315
62.9 815 333 335 345 48.8 1309
Alloy 25 75.4 795 287 304 319 56.1 1338
66.3 784 292 305 319 50.9 1279
75.8 798 293 307 321 56.3 1347
Alloy 26 56.5 1256 622 649 667 44.8 1882
55.9 1216 652 704 724 44.4 1824
56.9 1243 646 687 705 45.0 1885
Alloy 27 74.2 717 273 295 314 55.4 1188
71.9 727 282 305 324 54.1 1190
71.4 739 282 308 327 53.8 1210
Alloy 28 38.0 1251 613 638 648 32.2 1640
37.4 1253 599 627 638 31.7 1635
38.0 1251 610 639 651 32.2 1637
Alloy 29 38.6 1052 581 604 626 32.6 1457
44.7 1095 573 596 617 36.9 1580
42.2 1085 574 603 624 35.2 1543
Alloy 30 45.1 1304 455 496 522 37.2 1840
51.1 1287 472 512 538 41.2 1853
46.0 1282 460 498 525 37.9 1846
Alloy 31 44.9 1326 439 478 504 37.0 1824
43.6 1321 443 481 505 36.1 1810
49.5 1315 442 477 502 40.2 1893
Alloy 32 67.1 1027 551 574 592 51.3 1707
73.2 1048 571 587 603 54.9 1802
66.6 1051 574 590 605 51.0 1744
Alloy 33 67.1 1027 551 371 381 52.3 1319
73.2 1048 571 376 385 52.0 1322
66.6 1051 574 380 388 52.4 1342
Alloy 34 50.3 918 478 504 525 40.8 1332
53.4 918 477 507 529 42.8 1353
53.1 899 449 472 491 42.5 1324
Incremental Tensile Testing Incremental tensile testing was done on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control. Samples were tested at a displacement rate of 0.025 mm/s during initial loading to 2% strain and 0.125 mm/s for the remaining duration of the test. Due to the variation in sample length during testing effective strain rates generally ranged from ˜104/s to 10−3/s for the initial loading and after initial loading strain rates ranged from ˜10−3/s to ˜10−2/s. It should be noted that while the incremental tensile testing was done at these indicated strain rates, such incremental tensile testing is considered to support the yield strength distributions (i.e. values of A2, A3 and A4) and increase in magnetic phase volume for the alloys herein at the recited at strain rates (100/sec to 102/sec). See, e.g., Case Example #3 (stamping) and Table 13 (incremental tensile testing).
A control specimen from the same area of the sheet was tested up to failure from each alloy to evaluate initial sheet properties of the specific sample set used for incremental testing and the results are listed in Table 5 for each alloy herein. The ultimate tensile strength values are in a range from 745 to 1573 MPa with total elongation recorded in the range from 13.3 to 77.1%. The 0.5% proof stress or yield strength (A1) varies from 287 to 668 MPa and true ultimate tensile strength is in a range from 1175 to 2059 MPa. After each control specimen was tested, a new duplicate sample of each alloy was then strained approximately 5%, and then unloaded. The specimen dimensions were measured as well as the magnetic phases volume percent (Fe %) prior to the next increment of testing. Magnetic phases volume percent (Fe %) was measured by Fisher Feritscope.
TABLE 5
Tensile Properties of Alloys From Incremental Testing
True Difference
Total 0.5% Ultimate Ultimate Between True and
Elon- Proof Tensile Tensile Engineering
gation Stress Strength Strength Ultimate Tensile
(%) (MPa) (MPa) (MPa) Strength
Alloy D1 A1 B1 C1 (MPa)
Alloy 1 46.7 523 1166 1688 522
Alloy 2 30.9 350 1573 2059 486
Alloy 3 58.3 509 1140 1805 665
Alloy 4 35.4 450 1501 2029 528
Alloy 5 58.9 424 1112 1751 639
Alloy 6 13.3 503 1522 1701 179
Alloy 7 52.8 420 1223 1814 591
Alloy 8 57.9 440 1190 1863 673
Alloy 9 53.7 512 1001 1509 508
Alloy 10 62.5 449 1126 1781 655
Alloy 11 77.1 324 914 1618 704
Alloy 12 63.1 444 1150 1876 726
Alloy 13 42.5 360 1014 1428 414
Alloy 14 66.1 424 993 1649 656
Alloy 15 40.9 314 1199 1689 490
Alloy 16 38.7 378 1240 1720 480
Alloy 17 54.9 428 1126 1744 618
Alloy 18 50.9 287 1064 1606 542
Alloy 19 42.5 346 1386 1975 589
Alloy 20 72.6 465 869 1500 631
Alloy 21 65.7 483 999 1655 656
Alloy 22 65.2 485 1094 1807 713
Alloy 23 61.7 345 746 1206 460
Alloy 24 59.6 334 826 1318 492
Alloy 25 62.1 314 804 1303 499
Alloy 26 56.0 634 1214 1894 680
Alloy 27 57.7 325 745 1175 430
Alloy 28 34.4 668 1236 1661 425
Alloy 29 43.7 600 1080 1552 472
Alloy 30 48.6 490 1282 1905 623
Alloy 31 34.7 499 1295 1743 448
Alloy 32 54.0 579 1003 1545 542
Alloy 33 51.5 377 824 1248 424
Alloy 34 41.3 461 895 1265 370
Incremental test data for each alloy herein is listed in Table 6 through Table 39 and illustrated in FIG. 4 through FIG. 37. Sheet materials from alloys herein before testing have magnetic phases volume percent ranging from 0.2 to 40.7 Fe %. An increase in magnetic phases volume percent was observed in each alloy herein during incremental testing with difference between initial state and after the last cycle from 0.7 up to 83.3 Fe % depending on alloy chemistry. Incremental testing results also demonstrate a significant strengthening of the materials with increase in yield strength (0.5% proof stress). In all of the alloys herein from first cycle to the last one, more than 600 MPa increase in yield strength is found. Maximum difference in yield strength of 1750 MPa is recorded in Alloy 19. Since during forming, strengthening occurs to a lesser or greater extent in lower or more highly deformed localized area of the deformed part respectively, this will determine the magnitude of localized yield strength measured. As the incremental test data shows the initial undeformed strength levels and additionally the final strength until failure, sets the expected range of strengthening for a formed part for each alloy. The result of the incremental testing shown in Tables 6 through 39, clearly show a range of yield strengths are possible with the alloys here-in including the three identified distributions from the baseline value for each alloy; ±100 MPa, >100 to <600 MPa, and ≥600 MPa.
TABLE 6
Incremental Test Data For Alloy 1
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 0.7
1 4.3 476 517 548 1.6
2 8.6 571 674 686 4.9
3 12.9 668 764 770 12.2
4 17.4 807 871 889 22.4
5 22.0 1012 1062 1067 32.8
6 26.8 1223 1267 1279 43.3
7 31.6 1443 1449 1458 45.4
8 35.6 1555 1592 1611 56.1
9 41.2 1788 1760 1762 67.5
Change 1312 1243 1204 66.8
(Last Cycle − First Cycle)
TABLE 7
Incremental Test Data For Alloy 2
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 22.0
1 4.4 298 345 418 41.1
2 8.5 827 841 876 51.4
3 12.4 1255 1252 1290 59.1
4 16.3 1608 1616 1611 62.6
5 20.3 1868 1860 1890 64.4
6 21.0 2029 2043 — 67.1
Change 1731 1698 1472 45.7
(Last Cycle − First Cycle)
TABLE 8
Incremental Test Data For Alloy 3
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 0.9
1 4.5 491 506 525 1.1
2 8.9 677 686 700 1.1
3 13.2 818 840 849 1.1
4 17.5 880 975 982 1.1
5 22.1 996 1100 1107 1.3
6 26.6 1074 1211 1212 1.4
7 31.3 1150 1310 1307 1.5
8 36.4 1230 1388 1402 1.9
9 40.2 1335 1502 1497 2.0
10 46.3 1404 1570 1565 2.3
11 50.0 1496 1662 1662 2.3
12 52.2 1601 1767 1797 2.5
Change 1110 1261 1272 1.6
(Last Cycle − First Cycle)
TABLE 9
Incremental Test Data For Alloy 4
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 9.6
1 4.5 416 442 475 29.7
2 8.7 665 676 691 40.2
3 13.0 939 941 964 49.6
4 17.2 1194 1213 1214 54.8
5 21.5 1456 1497 1512 59.5
6 22.3 1691 — — 62.6
Change 1275 1055 1037 53.0
(Last Cycle − First Cycle)
TABLE 10
Incremental Test Data For Alloy 5
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 0.9
1 4.3 393 428 458 1.6
2 8.5 483 571 583 3.4
3 12.8 540 655 664 8.2
4 17.2 638 749 755 16.5
5 21.7 770 883 889 26.9
6 26.3 941 1060 1064 35.3
7 35.6 1120 1243 1249 45.3
8 37.5 1356 1470 1469 51.1
9 41.6 1518 1578 1575 54.5
10 46.9 1759 1782 1774 55.1
11 53.8 1843 1839 1830 58.1
Change 1450 1411 1372 57.2
(Last Cycle − First Cycle)
TABLE 11
Incremental Test Data For Alloy 6
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 40.7
1 2.2 379 543 798 63.4
2 4.2 1272 1298 1351 68.1
3 6.2 1542 1546 1553 70.6
4 8.1 1670 1656 1656 71.6
5 10.4 1801 1750 1699 73.0
Change 1422 1207 901 32.3
(Last Cycle − First Cycle)
TABLE 12
Incremental Test Data For Alloy 7
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.1
1 4.2 414 440 469 2.5
2 8.5 484 591 605 7.6
3 12.6 599 711 719 16.2
4 16.6 743 869 873 27.4
5 20.5 935 1067 1075 36.4
6 24.4 1131 1293 1289 46.0
7 28.1 1330 1466 1478 51.1
8 31.7 1541 1576 1638 56.5
9 35.3 1773 1735 1755 59.6
10 38.9 1944 1904 1848 59.6
11 40.2 1898 1729 — 63.0
Change 1484 1289 1379 61.9
(Last Cycle − First Cycle)
TABLE 13
Incremental Test Data For Alloy 8
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.2
1 4.2 393 418 447 2.4
2 8.4 481 599 619 5.5
3 12.5 570 715 728 11.9
4 16.4 678 863 872 20.0
5 20.3 804 1030 1031 28.0
6 24.1 947 1181 1190 35.5
7 27.8 1092 1341 1339 41.3
8 31.5 1248 1473 1462 47.4
9 35.1 1426 1596 1580 51.6
10 38.8 1614 1709 1694 57.4
11 40.1 1827 — — 62.7
Change 1434 1291 1247 61.5
(Last Cycle − First Cycle)
TABLE 14
Incremental Test Data For Alloy 9
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 0.9
1 4.4 497 517 536 1.2
2 8.7 660 679 691 1.2
3 12.9 754 811 819 1.2
4 16.9 829 923 930 1.2
5 21.0 902 1029 1035 1.2
6 24.9 978 1125 1130 1.4
7 28.8 1048 1215 1220 1.6
8 32.7 1128 1308 1315 1.8
9 36.5 1221 1411 1397 2.0
10 40.3 1320 1492 1498 2.2
11 44.0 1374 1540 1536 2.3
12 48.9 1472 1649 1637 2.8
Change 975 1132 1101 1.9
(Last Cycle − First Cycle)
TABLE 15
Incremental Test Data For Alloy 10
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.2
1 4.2 418 444 471 1.6
2 8.2 466 591 622 3.1
3 12.3 537 703 731 6.1
4 16.5 628 833 857 10.3
5 20.8 723 968 987 15.8
6 25.2 819 1096 1114 23.6
7 29.8 933 1240 1232 28.3
8 34.1 1043 1360 1337 34.7
9 38.9 1185 1448 1452 41.2
10 44.1 1342 1585 1575 42.2
11 50.5 1482 1664 1650 47.2
12 54.5 1660 1770 1795 52.3
Change 1242 1326 1324 51.1
(Last Cycle − First Cycle)
TABLE 16
Incremental Test Data For Alloy 11
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.2
1 4.7 318 323 334 1.6
2 9.3 450 453 465 1.6
3 13.8 568 572 579 1.7
4 18.2 675 679 686 1.8
5 22.5 783 793 799 2.1
6 26.8 888 887 902 2.5
7 31.0 985 994 997 2.8
8 35.0 1087 1087 1104 3.4
9 39.1 1181 1192 1190 3.7
10 43.0 1234 1248 1245 3.7
11 46.3 1329 1342 1342 3.7
Change 1011 1019 1008 2.5
(Last Cycle − First Cycle)
TABLE 17
Incremental Test Data For Alloy 12
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.5
1 4.2 402 430 460 2.1
2 8.3 467 579 597 4.6
3 12.6 542 682 695 9.9
4 16.9 651 807 816 17.7
5 21.3 824 962 970 27.3
6 26.1 990 1129 1142 36.5
7 30.8 1113 1300 1313 42.4
8 35.2 1267 1439 1463 48.3
9 40.5 1483 1555 1569 52.2
10 47.7 1701 1747 1736 59.7
Change 1299 1317 1276 58.2
(Last Cycle − First Cycle)
TABLE 18
Incremental Test Data For Alloy 13
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.7
1 4.4 341 368 402 9.5
2 8.7 498 540 547 21.6
3 13.0 655 697 704 35.8
4 17.1 833 863 875 47.5
5 21.2 1009 1022 1030 54.8
6 25.2 1172 1169 1173 62.0
7 29.2 1289 1268 1276 69.2
8 32.5 1487 1443 1452 70.5
Change 1146 1075 1050 68.8
(Last Cycle − First Cycle)
TABLE 19
Incremental Test Data For Alloy 14
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 2.0
1 4.4 400 431 459 2.0
2 8.5 538 589 600 2.3
3 12.8 589 701 712 3.1
4 17.2 652 805 817 4.6
5 21.6 726 911 923 6.9
6 26.2 812 1022 1033 9.7
7 30.8 891 1132 1146 13.4
8 35.7 980 1243 1246 16.1
9 40.0 1067 1330 1343 19.1
10 44.4 1162 1429 1430 20.4
11 50.3 1268 1568 1548 23.3
12 53.0 1341 1644 1622 27.6
Change 941 1213 1163 25.7
(Last Cycle − First Cycle)
TABLE 20
Incremental Test Data For Alloy 15
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 2.4
1 4.5 304 321 357 14.0
2 8.9 492 511 517 31.9
3 13.2 721 728 752 47.9
4 17.3 984 1003 1008 58.0
5 21.4 1245 1249 1241 64.7
6 25.3 1439 1429 1420 69.5
7 30.2 1550 1571 1545 71.7
Change 1246 1250 1188 69.4
(Last Cycle − First Cycle)
TABLE 21
Incremental Test Data For Alloy 16
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.6
1 4.5 349 373 392 16.0
2 8.8 430 448 462 38.2
3 13.2 648 674 696 56.6
4 17.5 998 1015 1028 67.2
5 22.0 1305 1306 1312 75.2
6 26.7 1490 1510 1505 79.1
7 31.7 1647 1626 1622 83.3
Change 1298 1253 1230 81.7
(Last Cycle − First Cycle)
TABLE 22
Incremental Test Data For Alloy 17
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.9
1 4.2 390 425 470 6.2
2 8.3 555 653 664 13.0
3 12.5 645 797 806 22.2
4 17.0 769 949 954 31.9
5 21.5 889 1079 1082 39.5
6 26.2 1034 1191 1224 46.4
7 31.3 1195 1309 1320 54.4
8 38.1 1500 1487 1484 55.9
9 41.7 1569 1592 1587 61.3
10 45.5 1745 1705 1671 65.1
Change 1355 1280 1201 63.2
(Last Cycle − First Cycle)
TABLE 23
Incremental Test Data For Alloy 18
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 3.6
1 4.6 310 312 320 26.8
2 9.0 532 535 561 38.6
3 13.5 783 784 793 45.5
4 18.1 962 965 978 51.3
5 22.9 1124 1119 1132 54.7
6 27.6 1258 1244 1246 58.3
7 32.8 1394 1379 1373 61.2
8 37.9 1498 1507 1517 62.8
9 43.7 1578 1574 1548 67.1
Change 1268 1262 1228 63.5
(Last Cycle − First Cycle)
TABLE 24
Incremental Test Data For Alloy 19
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 16.3
1 4.5 340 358 392 31.0
2 8.8 642 655 677 38.7
3 13.2 879 889 918 45.7
4 17.6 1138 1146 1160 51.6
5 21.9 1420 1405 1443 56.0
6 26.2 1649 1628 1644 58.1
7 30.8 1864 1848 1847 61.3
8 35.1 2034 1983 2037 61.7
9 38.8 2107 2108 2082 64.5
Change 1767 1750 1690 48.2
(Last Cycle − First Cycle)
TABLE 25
Incremental Test Data For Alloy 20
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.4
1 4.4 458 478 499 1.9
2 8.6 556 609 620 1.9
3 12.9 613 705 714 2.0
4 17.3 676 788 797 2.3
5 21.9 727 866 875 3.1
6 26.6 791 944 951 4.5
7 31.4 883 1018 1025 6.4
8 36.4 1012 1098 1103 8.7
9 41.6 1011 1170 1179 12.4
10 47.1 1084 1265 1258 15.2
11 51.8 1145 1318 1312 20.2
12 58.4 1290 1485 1492 23.8
13 65.2 1361 1518 1511 23.6
14 69.0 1502 1538 1530 27.9
15 72.5 1571 1694 1676 30.5
Change 1113 1216 1177 29.1
(Last Cycle − First Cycle)
TABLE 26
Incremental Test Data For Alloy 21
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.4
1 4.2 452 484 510 1.8
2 8.3 509 619 633 2.1
3 12.6 556 703 721 3.3
4 17.0 617 783 797 6.3
5 21.7 700 870 882 11.1
6 26.4 802 968 978 16.9
7 31.1 922 1088 1097 24.9
8 36.1 1040 1212 1209 30.5
9 41.2 1204 1323 1327 36.1
10 45.1 1311 1456 1435 36.1
11 51.1 1462 1572 1587 43.4
12 56.9 1629 1679 1671 48.1
13 59.5 1713 1734 1732 48.1
Change 1261 1250 1222 46.7
(Last Cycle − First Cycle)
TABLE 27
Incremental Test Data For Alloy 22
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number [%] [MPa] [MPa] [MPa] [Fe %]
0 0.0 — — — 1.4
1 4.1 446 482 511 1.8
2 8.2 495 617 634 2.4
3 12.4 572 707 720 5.1
4 17.0 631 794 802 10.6
5 21.7 742 902 907 18.3
6 26.4 866 1040 1043 27.1
7 31.2 1041 1181 1201 35.2
8 36.1 1224 1343 1359 39.7
9 40.4 1323 1435 1493 45.8
10 46.6 1500 1586 1582 50.0
11 48.0 1769 — — 54.6
Change 1323 1104 1071 53.2
(Last Cycle − First Cycle)
TABLE 28
Incremental Test Data For Alloy 23
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 1.0
1 4.7 344 347 362 1.4
2 9.3 465 469 478 1.4
3 13.9 577 579 585 1.3
4 18.4 674 675 679 1.3
5 22.8 761 760 762 1.2
6 27.2 828 827 828 1.1
7 31.5 860 858 860 1.1
8 35.8 998 995 996 1.1
9 40.0 1037 1032 1034 1.1
10 44.2 1103 1098 1100 1.1
11 48.7 1158 1149 1148 1.5
Change 814 802 786 0.5
(Last Cycle − First Cycle)
TABLE 29
Incremental Test Data For Alloy 24
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 1.3
1 4.7 333 337 347 1.5
2 9.3 458 462 472 1.4
3 13.9 574 576 583 1.4
4 18.4 676 678 682 1.4
5 22.7 772 776 777 1.3
6 27.0 866 866 871 1.3
7 31.3 955 957 957 1.4
8 35.4 1014 1031 1032 1.4
9 39.6 1121 1124 1127 1.6
10 43.6 1183 1184 1204 1.7
11 47.2 1222 1230 1224 1.8
Change 889 893 877 0.5
(Last Cycle − First Cycle)
TABLE 30
Incremental Test Data For Alloy 25
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 1.0
1 4.7 301 309 323 1.3
2 9.3 437 444 454 1.4
3 13.7 555 563 571 1.4
4 18.1 652 666 672 1.7
5 22.4 739 763 767 2.1
6 26.6 814 851 854 2.8
7 30.8 894 941 944 3.6
8 34.9 970 1032 1031 4.5
9 39.0 1040 1110 1106 5.6
10 43.0 1098 1175 1167 5.5
11 45.4 1176 1231 1228 8.3
Change 875 922 905 7.3
(Last Cycle − First Cycle)
TABLE 31
Incremental Test Data For Alloy 26
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 13.2
1 4.2 603 639 659 16.6
2 8.3 604 749 762 18
3 12.5 669 815 818 23.8
4 16.6 784 870 876 33.4
5 20.6 941 1014 1016 41.55
6 24.5 1153 1215 1228 49.45
7 28.3 1377 1444 1453 54.4
8 32.0 1574 1622 1630 54.1
9 35.7 1772 1770 1760 61.2
10 37.5 1874 — — 63.3
Change 1271 1131 1101 50.1
(Last Cycle − First Cycle)
TABLE 32
Incremental Test Data For Alloy 27
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 1.2
1 4.6 313 326 343 1.7
2 9.1 449 459 469 1.9
3 13.6 552 568 576 2.2
4 17.9 638 664 670 2.7
5 22.2 719 753 758 3.9
6 26.4 809 840 843 5.1
7 30.6 872 920 922 6.3
8 34.7 942 996 997 7.7
9 38.9 1001 1055 1059 10.1
10 43.0 1100 1148 1143 12.0
11 44.2 1200 1229 — 14.6
Change 887 903 800 13.4
(Last Cycle − First Cycle)
TABLE 33
Incremental Test Data For Alloy 28
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 17.4
1 4.3 631 657 669 22.5
2 8.7 660 720 727 37.6
3 13.0 730 732 741 48.2
4 17.1 927 960 973 58.5
5 21.1 1263 1293 1297 66.1
6 25.0 1517 1539 1532 72.2
7 28.7 1691 1679 1665 72.0
Change 1060 1022 996 54.6
(Last Cycle − First Cycle)
TABLE 34
Incremental Test Data For Alloy 29
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 2.0
1 4.2 573 598 621 2.6
2 8.3 625 741 756 3.1
3 12.4 667 833 850 4.9
4 16.3 725 917 932 8.6
5 20.3 811 1009 1022 13.6
6 24.2 903 1115 1128 21.1
7 28.6 1029 1248 1255 30.1
Change 456 650 634 27.5
(Last Cycle − First Cycle)
TABLE 35
Incremental Test Data For Alloy 30
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 2.3
1 4.3 456 491 517 3.4
2 8.6 494 598 605 8.4
3 12.9 571 644 648 20.8
4 17.1 716 767 774 35.5
5 21.0 966 1014 1024 46.6
6 24.9 1258 1317 1322 55.6
7 27.7 1509 1523 1524 59.9
Change 1053 1032 1007 57.6
(Last Cycle − First Cycle)
TABLE 36
Incremental Test Data For Alloy 31
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 2.9
1 4.4 435 469 495 5.8
2 8.8 495 566 567 17.4
3 13.1 583 622 625 32.8
4 17.2 783 819 835 47.3
5 19.7 1099 1145 1156 55.3
Change 664 676 661 52.4
(Last Cycle − First Cycle)
TABLE 37
Incremental Test Data For Alloy 32
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 1.0
1 4.4 563 578 596 1.9
2 8.6 692 728 739 1.9
3 12.7 751 841 849 1.8
4 16.8 809 937 945 2.1
5 20.8 871 1025 1032 2.6
6 24.7 939 1113 1120 3.5
7 28.6 1005 1195 1202 4.8
8 33.4 1071 1280 1287 10.8
Change 508 702 691 9.8
(Last Cycle − First Cycle)
TABLE 38
Incremental Test Data For Alloy 33
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 0.7
1 4.6 378 383 393 0.8
2 9.2 509 515 527 0.9
3 13.6 631 639 648 0.9
4 17.9 736 751 757 0.9
5 22.2 823 848 852 0.9
6 26.4 903 938 940 0.9
7 30.6 973 1017 1018 0.9
8 34.7 1042 1092 1093 1.0
9 38.8 1099 1153 1155 1.0
10 42.8 1175 1246 1241 1.0
11 46.9 1259 1324 1317 1.4
12 49.8 1404 1464 1445 1.5
Change 1026 1081 1052 0.8
(Last Cycle − First Cycle)
TABLE 39
Incremental Test Data For Alloy 34
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) (MPa) (MPa) (MPa) (Fe %)
0 0.0 — — — 0.4
1 4.5 459 481 501 0.5
2 8.9 649 659 671 0.6
3 13.3 777 797 804 0.6
4 17.5 881 913 918 0.6
5 21.7 973 1016 1019 0.6
6 25.8 1053 1109 1110 0.6
7 29.9 1134 1197 1196 0.5
8 33.3 1292 1363 1351 0.7
Change 833 882 850 0.3
(Last Cycle − First Cycle)
As can be seen from the above, the magnetic phases volume of the sheet is increased when exposed to one or a plurality of strains above the yield strength of the sheet. That is, for a given sheet material, having a magnetic phases volume that falls in the range of 0.2 Fe % to 45.0 Fe %, such value is observed to increase and the metal part that is formed indicates a magnetic phases volume that falls in the range of 0.5 Fe % to 85.0 Fe %. For example, for Alloy 1 that indicates in the sheet an initial magnetic phase volume of 0.7 Fe %, after nine (9) strains above the yield strength of the sheet indicates a magnetic phases volume of 67.5 Fe %. Alloy 2 sheet is initially 22.0 Fe % and after six (6) strains above the yield strength of the sheet indicates a magnetic phases volume of 67.1 Fe %. For each alloy provided herein, the properties including yield change as a function of applied strain in sheet form. In stamping operations, a wide range of strains rather than a singular strain is applied over the stamped part. This results in a wide range of localized strain and resulting properties in the stamped part which may include the entire range of properties found for example by the separately applied strains in the sequential cycles for each alloy.
Case Examples Case Example #1 Structural Changes During Cold Deformation These results show the key structural changes which lead to strengthening during cold deformation with commensurate increases in both yield and tensile strength during the deformation process.
Laboratory slabs with thickness of 50 mm were cast from Alloy 7 and Alloy 8 according to the atomic ratios in Table 1 that were then laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described in the Main Body section of the current application. Microstructure of the alloys in a form of processed sheet with 1.2 mm thickness after annealing corresponding to a condition of the sheet in annealed coils at commercial production was examined by SEM and TEM.
To prepare TEM specimens for a structural analysis of the annealed sheet from the alloys before deformation, the samples were first cut with EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to make foils of 60 to 70 μm thickness was done by polishing with 9 μm, 3 μm, and 1 μm diamond suspension solution, respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled with electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling usually is done at 4.5 keV, and the inclination angle is reduced from 4° to 2° to open up the thin area. To analyze structure in the alloys after deformation, TEM samples were cut from the gauge section of the tensile specimens close to the fracture and prepared in the similar manner. The TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV. The TEM specimens were studied by SEM. Microstructures were examined by SEM using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
The microstructure in the Alloy 7 sheet before deformation is shown by SEM and TEM micrographs in FIGS. 38a and b, respectively. The microstructure consists primarily of recrystallized micron-sized austenite grains, 1 to 10 μm in size, containing annealing twins and stacking faults. Annealing twins are generally understood as a highly symmetrical interface within one crystal or grain and form during annealing. Stacking faults are a more general term to describing an interruption of the normal stacking sequence of atomic planes in a crystal or grain. Detailed analysis of the structure also reveals a small fraction of ferrite (<1%) and the presence of isolated nanoprecipitates typically in the 5 to 100 nm size range (FIG. 38c). Similar structure was observed in the Alloy 8 sheet before deformation shown in FIG. 39. Detailed analysis of the structure also reveals a small fraction of ferrite (<1%) and the presence of isolated nanoprecipitates typically in the 5 to 100 nm size range (FIG. 38c). Similar structure was observed in the Alloy 8 sheet before deformation shown in FIG. 39.
During tensile testing to failure, the initial structure undergoes NR&S leading to formation of the final structure, which is demonstrated for Alloy 7 and Alloy 8 by SEM and TEM micrographs in FIG. 40 and FIG. 41, respectively. As can be seen, the structure after deformation is much different than the starting structure and consists of two distinct microstructural regions of Microconstituent 1 and Microconstituent 2 as shown in FIG. 40b and FIG. 41b.
Further details of the microstructure after deformation highlighting microstructural features of each microconstituent were obtained from structural analysis of the gauge section of the tensile specimen from Alloy 8 sheet after testing to failure. A TEM bright-field micrograph corresponding to Microconstituent 1 in the sheet material is shown in FIG. 42a. Microconstituent 1 is a result of phase transformation during cold deformation and characterized by refined ferrite, with grain sizes from 20 to 750 nm, and nanoprecipitates. Its formation can be quantified by measurement of magnetic phases volume percent (Fe %) using Feritscope as demonstrated for alloys herein during incremental testing (see Main Body). In the case of Alloy 8 sheet, before deformation it has less than 1 Fe % of magnetic phases volume percent as measured by the Feritscope. After tensile testing to failure, measured value near the fracture is about 62.7 Fe %. Microconstituent 1 is found to contain significant volume fractions (˜4 vol %) of nanoprecipitates typically from 2 to 20 nm in diameter although larger nanoprecipitates can be occasionally found up to 100 nm in size. In FIG. 42b, a TEM dark-field micrograph of the Microconstituent 1 area illustrates the nanoscale ferrite grains that are typically from 150 to 300 nm in size and formed as a result of transformation from austenite during the deformation process. After transformation, the nanoscale ferrite is also found to participate in the deformation process through dislocation mechanisms. In FIG. 42c, a TEM dark-field micrograph shows a selected nanoscale ferrite grain at higher resolution. As shown, this grain contains a high density of dislocations, which form with a tangled morphology indicating that after formation, this grain continued to deform and contribute to the measured total elongation. Thus, the NR&S mechanism leading to structural evolution during cold deformation described above involves complex interaction of dislocation dominated deformation mechanisms along with phase transformation (e.g. austenite to ferrite), nanoscale phase formation (e.g. creation of nanoferrite from 20 nm to 750 nm), nanoprecipitation and results in material strengthening confirmed by the yield strength distributions identified in FIG. 2. HREM image of the nanoprecipitate examples are shown in FIG. 42d.
A TEM bright-field micrograph corresponding to Microconstituent 2 in the sheet material is shown in FIG. 43a. Microconstituent 2 is represented by micron-sized un-transformed austenite and nanoprecipitates with high dislocation density and dislocation cell formation after deformation (FIG. 43b). Microconstituent 1 is also found to contain nanoprecipitates that are highlighted by circles in FIG. 43c and are typically from 2 to 20 nm in diameter although larger nanoprecipitates can be occasionally found up to 100 nm in size. In FIG. 43d, a HREM image of the nanoprecipitate example is shown.
This Case Example demonstrates that the microstructure of the alloys herein undergo transformation during cold deformation through the NR&S mechanism leading to formation of the microstructure with distinct microconstituents resulting in material strengthening.
Case Example #2 Nondestructive Analysis of Stamped Part Sheet blanks from Alloy 8 with a thickness of 1.4 mm were used for stamping trial of a B-pillar at a commercial stamping facility with stamping speed estimated at 290 mm/s. Using an existing die, Alloy 8 sheet blanks were stamped into B-pillars. Non-destructive analysis of the B-pillar was done by Feritscope measurements of the local magnetic phases volume percent in different areas.
Feritscope measurements provide an indication of the structural changes occurring during deformation from stamping. As shown previously, in the Alloy 8 sheet, the initial sheet microstructure changes from non-magnetic (i.e. paramagnetic) to magnetic (i.e. ferromagnetic) microstructure during cold deformation through the NR&S mechanism. The baseline for the sheet in Feritscope measurements before stamping was <1 Fe %. Increase in the volume fraction of Microconstituent 1 results in higher Fe % measured. Feritscope measurements with ˜20 mm grid pattern were taken from two stamped B-pillars including one which underwent 4 out of 5 stamping hits and one which underwent 5 out of 5 stamping hits. The 5th hit is mainly a flanging operation so little structural or property change was expected in the B-pillar. The examples of the grid pattern on the different areas of the B-pillars are shown in FIG. 44.
The summary of Fe % measurements of the B-pillar which underwent a total of 4 stamping hits is shown in FIG. 45. Note that out of the 1426 total measurements taken, 487 of these measurements remained at <1 Fe % and are not shown in FIG. 45 as in these areas, little or no strain was imposed on the sheet during stamping so it remained at its baseline value. In FIG. 46, a histogram of the Feritscope measurements on the B-pillar which underwent all 5 stamping operations is shown. In a similar fashion, out of the 1438 total measurements taken, 510 of these were still at the baseline sheet value and are not shown. Analysis of the data shows that in approximately ˜65% of the areas measured, increase in Fe % corresponding to nano-ferrite formation and indicating strengthening through the NR&S mechanism was observed. The fraction of the stamping which undergoes strengthening will depend on the amount of material deformed during the stamping operation, which is highly dependent on the localized strain (i.e. amount of deformation which occurs in a particular area of a deformed part). Additionally, for both stamped B-pillars, the highest magnetic phases volume percent measured was 31 Fe % measured in the most deformed areas. Thus, the 1438 measurements show a wide range of Fe % numbers at each localized area from <1% to 31 Fe %. This clearly shows localized structural changes and this is then expected to be concurrent with localized yield strength changes leading to three distinct yield strength distributions.
This Case Example demonstrates significant changes in magnetic phases volume percent in the stamping as compared to initial sheet. These changes correspond to microstructural transformation the unique NR&S mechanisms leading to sheet material strengthening as it deforms.
Case Example #3 Destructive Analysis of Stamped Part A sheet blank from Alloy 8 with a thickness of 1.4 mm were used for a stamping trial of a B-pillar at a commercial stamping facility with stamping speed estimated at 290 mm/s. Alloy sheet properties before stamping are shown in Table 40. Using an existing die, Alloy 8 sheet blanks were stamped into B-pillars.
TABLE 40
Average Tensile Properties Of 1.4 mm Thick Alloy 8 Sheet
Magnetic
Ultimate 0.2% 0.5% Phases
Tensile Proof Proof Total Rockwell C Volume
Strength Stress Stress Elongation Hardness Percent
[MPa] [MPa] [MPa] [%] [HRC] [Fe %]
1173 460 525 57.3 22.1 0.2
For destructive analysis, tensile specimens were cut along the entire length of the B-pillar. The view of the B-pillar before and after specimen cutting is shown in FIG. 47. Tensile specimens with reduced size (i.e. 12.5 mm gauge) were used to evaluate material properties in the stamped part. Property values measured for reduced size specimens were shown to be in good correlation with that measured during testing of ASTM E8 standard specimens. Such property correlation for Alloy 8 is shown in FIG. 48.
In total, 213 tensile specimens cut from the B-pillar were tested. Rockwell C hardness and Feritscope measurements were taken from each tensile specimen. Tensile property data for selected specimens are listed in Table 41. Examples of the stress—strain curves for specimens cut from the B-pillar with various levels of magnetic phases volume percent (Fe %) are presented in FIG. 49. Corresponding true stress—true strain curves in FIG. 50 show extensive strain hardening in the material indicating the effect of NR&S on the sheet structure and properties during stamping.
TABLE 41
Tensile Properties of Selected Specimens Cut From The Stamping
0.2% 0.5% 1.0% Ultimate
Magnetic Phases Proof Proof Proof Tensile Total Rockwell C
Volume Percent Stress Stress Stress Strength Elongation hardness
[Fe %] [MPa] [MPa] [MPa] [MPa] [%] [HRC]
0.2 450 521 564 1182 61.7 27
2.1 545 634 685 1222 57.3 32
4.6 503 652 733 1212 54.2 32
9.0 621 774 869 1231 46.7 38
13.9 716 896 992 1326 39.1 39
20.2 787 1007 1147 1320 37.0 46
24.5 954 1229 1327 1410 27.0 46
The measured tensile properties were correlated to structural changes during stamping evaluated from direct Feritscope measurements on the grip sections of the tensile specimens after cutting from the B-pillar prior to testing. Correlation between the measured Fe % and tensile properties is shown in FIG. 51a for strength characteristics and in FIG. 51b for total elongation demonstrating linear relationships.
Non-destructive analysis showed the maximum value of 31 Fe % in highly bent areas of the B-pillar that cannot be used for tensile specimen cutting. However, the current correlations based on 213 data points and shown in FIGS. 51a and b allows estimation of the strength characteristics and retained ductility in these areas by extrapolation of the linear relationships to 31 Fe % as shown in FIGS. 52a and b. At the maximum value of 31 Fe %, the 0.2% proof stress is estimated at 1085 MPa, 0.5% proof stress at 1400 MPa, and ultimate tensile strength at 1490 MPa. The amount of increase in 0.5% proof stress and ultimate tensile strength in most deformed areas of the stamped B-pillar over the baseline in Table 40 is estimated to be 875 MPa and 317 MPa, respectively. The retained ductility is estimated by the total elongation at about 15% in the most deformed areas of the B-pillar after stamping. These results indicate that the material has a potential for applications requiring stamping of even more complex geometries and the resulting stamped parts retain capability for high energy absorption.
This Case Example demonstrates a dramatic increase in both yield and tensile strength in the stamped part as a result of material cold deformation during stamping operation. Cold deformation activates NR&S mechanism in the alloys herein leading to material strengthening. The 213 tensile specimens measured over the surface of the stamped part illustrate the resulting change in properties resulting from the localized changes found in the stamped part. While the stamped part was not deformed until failure, the range of properties found in the stamped part, are similar to the range of tensile properties (prior to failure) found for the same alloy from incremental tensile testing as previously provided in Table 13.
Case Example #4 Microstructural Analysis of the Stamped Part A sheet blank from Alloy 8 with a thickness of 1.4 mm was used for stamping trial of a B-pillar at a commercial stamping facility. Detailed TEM analysis was done on the samples cut from different locations of the stamped part to demonstrate the structural response to the deformation during stamping.
To prepare TEM specimens for a structural analysis, the samples were first cut with EDM from the areas of interest, and then thinned by grinding with pads of reduced grit size every time. Further thinning to make foils of 60 to 70 am thickness was done by polishing with 9 am, 3 am, and 1 am diamond suspension solution, respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled with electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling usually is done at 4.5 keV, and the inclination angle is reduced from 4° to 2° to open up the thin area. To analyze structure in the alloys after deformation, TEM samples were cut from the gauge section of the tensile specimens close to the fracture and prepared in the similar manner. The TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV. The TEM specimens were studied by SEM. Microstructures were examined by SEM using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
FIG. 53 shows the bright-field TEM images of the microstructure in the selected samples cut from the stamped B-pillar before and after tensile testing. Analyzed samples were selected with 4.6 Fe %, 13.9 Fe %, and 24.5 Fe % of magnetic phases volume percent. Corresponding tensile properties and stress-strain curves for the selected specimens were shown earlier in Case Example #3 (Table 41, FIG. 49 and FIG. 50).
In FIG. 53a, c, and e, the microstructure corresponding to that in the as stamped part is shown at the three levels of deformation. In FIG. 53a, the microstructure of the sample (with 4.6% Fe) is slightly deformed where grain boundaries are still clearly visible since the material transformation is limited and only moderate amount of dislocations are generated in the grains. In FIGS. 53c and e, TEM images show an increase in the volume percent of Microconstituent 1 with higher dislocation density and some twins observed in both microconstituents. Through studying multiple locations, a clear correlation is found with the amount of activated NR&S occurring during stamping with increases of Fe % in the samples.
TEM analysis of the microstructure was also done for the gauge section of the corresponding samples tested in tension from the same three locations. Bright-field TEM images of the microstructure after tensile testing are provided in FIG. 53b, d, and f. It can be seen that after testing to failure, the structures in all three samples are similar with formation of distinct Microconstituent 1 and 2 regions as a result of further structural transformation through the NR&S mechanism during tensile testing. Structural evolution during tensile testing is also confirmed by Feritscope measurements showing 38 to 43 Fe % in the gauge of all tested samples.
This Case Example demonstrates microstructural changes of the alloy herein during stamping operations corresponding to localized increases in magnetic phases volume percent consistent with the localized Feritscope measurements. These specific microstructural changes are consistent with the activation of the identified NR&S mechanism and conclusively show the material strengthening occurring in the stamping.
Case Example #5 Correlation Between Incremental Tensile Testing and Destructive Analysis of Stamped Part Nine specimens with reduced size were cut from the same Alloy 8 sheet that used for stamping trial of the B-pillar and used for incremental testing. Alloy sheet properties are shown in Table 40. Incremental tensile testing was done on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control. The specimen dimensions were measured as well as the magnetic phases volume percent (Fe %) prior to next increment of testing. Magnetic phases volume percent (Fe %) was measured by Fisher Feritscope.
Yield strength data collected from incremental testing of Alloy 8 sheet as well as that from tensile testing of specimens cut from the B-pillar during destructive analysis were correlated with magnetic phases volume percent (Fe %). 0.2 and 0.5% proof stress as a function of the Fe % is presented in FIG. 54. Both characteristics are shown to increase with increasing Fe % in a linear manner.
This Case Example shows good correlation between the changes in yield strength in incremental tensile specimens and that in specimens tested during destructive analysis of the B-pillar as a function of magnetic phases volume percent. Cold deformation results in structural transformation detected by an increase in Fe % leading to strengthening of alloys herein and to an increase in strength characteristic values.
Case Example #6 Properties of Alloys 8 at Variable Thickness Laboratory slabs with thickness of 50 mm were cast from Alloy 8 according to the atomic ratios in Table 1. The slabs were then processed by a mixture of hot and cold rolling to achieve the targeted sheet thickness of 0.5, 1.3, 3.0 and 7.1 mm. The thickest material was hot rolled only, while all other conditions were cold rolled to achieve the targeted thickness. After cold rolling the samples were wrapped in stainless steel foil to minimize oxidation and placed into an 850° C. furnace for 10 minutes then removed and allowed to cool in air. The details of each sheet processing are listed in Table 42.
TABLE 42
Details Of Processing Towards Targeted Alloy 8 Sheet Thicknesses
First Second First Second Third
Sheet Hot Hot Cold Cold Cold
Thickness Rolling Rolling Rolling Rolling Rolling
[mm] [%] [%] [%] [%] [%]
0.5 80.5 84.9 25.2 31.1 30.5
1.3 80.6 77.6 40 — —
3.0 80.7 55.5 28.3 — —
7.1 78.6 32.3 — — —
Incremental tensile testing was done on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control. Samples were tested at a displacement rate of 0.025 mm/s during initial loading to 2% strain and 0.125 mm/s for the remaining duration of the test.
Each specimen was strained approximately 5%, and then unloaded. The specimen dimensions were measured as well as the magnetic phases volume percent (Fe %) prior to the next increment of testing. Magnetic phases volume percent (Fe %) was measured by Fisher Feritscope. Control specimen from the same sheet from each alloy was tested up to failure to evaluate initial sheet properties that are listed in Table 43 for sheet samples at each thickness.
TABLE 43
Tensile Properties Of Alloy 8 Sheet With Different Thicknesses
Ultimate
Sheet Total Tensile 0.5% Proof True True
Thickness elongation Strength Stress Strain Stress
[mm] [%] [MPa] [MPa] [%] [MPa]
0.5 54.1 1177 483 43.1 1799
1.3 57.9 1190 440 45.7 1862
3.0 60.2 1156 470 47.1 1823
7.1 47.8 1130 309 39.2 1662
Incremental test data for samples with each thickness herein is listed in Table 44 through Table 47. Incremental stress-strain curves along with engineering stress-strain curves and true stress-true strain curves are shown for Alloy 8 sheet with each thickness in FIG. 55a, FIG. 56a, FIG. 57a, and FIG. 58a. Good agreement between calculated true stress-true strain curve and incremental test data was observed in all cases. Yield strength and magnetic phases volume percent (Fe %) as a function of accumulated strain during incremental testing are plotted in FIG. 55b, FIG. 56b, FIG. 57b, and FIG. 58b for Alloy 8 sheet with 0.5, 1.3, 3.0, and 7.1 mm thickness, respectively. Sheet materials from Alloy 8 processed by cold rolling and annealing (0.5, 1.3 and 3.0 mm thickness) before testing have magnetic phases volume percent ranging from 1.2 to 1.6 Fe %. Alloy 8 sheet in hot rolled condition (7.1 mm thick) has magnetic phases volume percent of 3.1 Fe % before testing. After testing, there is a significant increase in Fe % in all cases resulting in final Fe % values from 43.5 to 62.7 Fe %.
The incremental testing results also show an extensive increase in yield strength with increasing accumulated strain. The difference in yield strength values between first and last cycle of testing varies from 1112 to 1332 MPa confirming a significant material strengthening. Note that while this example highlights individual strains applied to the sheet in specific steps, the range of properties demonstrated are deemed simultaneously possible in a stamped part made from the alloys herein.
TABLE 44
Incremental Test Data For Alloy 8 Sheet With 0.5 mm Thickness
Cumulative 0.5% Proof Magnetic
Cycle Applied Stress Phases Volume
Number Strain [%] [MPa] Percent [Fe %]
0 0 — 1.2
1 4.2 480 1.2
2 8.2 625 1.9
3 12.8 765 4.2
4 17.3 893 8.3
5 21.6 1041 13.8
6 26.1 1178 19.9
7 30.6 1326 27.9
8 35.3 1440 34.2
9 40.8 1598 37.5
10 44.0 1704 43.5
Change 1224 42.3
(Last Cycle − First Cycle)
TABLE 45
Incremental Test Data For Alloy 8 Sheet With 1.3 mm Thickness
Cumulative 0.5% Proof Magnetic
Cycle Applied Stress Phases Volume
Number Strain [%] [MPa] Percent [Fe %]
0 0 — 1.2
1 4.2 448 2.4
2 8.8 599 5.5
3 13.0 715 11.9
4 17.3 863 20.0
5 21.6 1030 28.0
6 26.1 1181 35.5
7 30.5 1341 41.3
8 35.1 1473 47.4
9 39.5 1596 51.6
10 46.5 1709 62.7
Change 1291 60.3
(Last Cycle − First Cycle)
TABLE 46
Incremental Test Data For Alloy 8 Sheet With 3.0 mm Thickness
Cumulative 0.5% Proof Magnetic
Cycle Applied Stress Phases Volume
Number Strain [%] [MPa] Percent [Fe %]
0 0 — 1.6
1 4.2 477 2.4
2 8.2 617 5.5
3 12.4 735 11.2
4 16.7 874 19.8
5 21.0 1027 28.8
6 25.4 1181 37.6
7 29.9 1321 43.9
8 34.5 1451 50.4
9 38.8 1593 52.3
10 44.4 1709 55.0
11 45.9 1799 58.4
Change 1332 56.0
(Last Cycle − First Cycle)
TABLE 47
Incremental Test Data For Alloy 8 Sheet With 7.1 mm Thickness
Cumulative 0.5% Proof Magnetic
Cycle Applied Stress Phases Volume
Number Strain [%] [MPa] Percent [Fe %]
0 0 — 3.1
1 4.35 330 10.1
2 8.54 520 18.7
3 12.8 664 28.7
4 17.14 819 35.2
5 21.5 977 41.8
6 26.02 1143 47.5
7 30.53 1287 51.6
8 35.33 1442 55.7
Change 1112 45.6
(Last Cycle − First Cycle)
This Case Example demonstrates that the strengthening and strain hardening mechanisms occur in the sheet material with a range of thicknesses from 0.5 to 7.1 mm.
Case Example #7 Incremental Testing of Sheet from Commercial Steel Grades Sheet material from commercial steel grades of TRIP 780 and DP980 was used for incremental testing. TRIP 780 has the following chemistry (at %); 97.93 Fe, 1.71 Mn, 0.15 Cr, 0.12 Si, 0.05 C, and 0.04 Cu. DP980 has the following chemistry (at %); 96.86 Fe, 2.34 Mn, 0.42 C, and 0.38 Si. Incremental tensile testing was done on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control. Samples were tested at a displacement rate of 0.025 mm/s during initial loading to 2% strain and 0.125 mm/s for the remaining duration of the test.
Each specimen was strained approximately 5%, and then unloaded. The specimen dimensions were measured prior to the next increment of testing. Control specimen from the same sheet from each steel grade was tested up to failure to evaluate initial sheet properties that are listed in Table 48 for each grade. Magnetic phases volume percent (Fe %) in initial sheet and in the specimen gauge after testing was measured by Fisher Feritscope that is listed in Table 49. The measurement showed no changes in Fe % before and after testing the specimens from TRIP 780 and DP980.
TABLE 48
Summary Of Average Properties Of Commercial Steel Grades
Difference
True 0.2% 0.5% 1.0% Ultimate Between
Total Strain at Proof Proof Proof Tensile True UTS and
Steel Elongation Fracture Stress Stress Stress Strength Stress True Stress
Grade (%) (%) [MPa] [MPa] [MPa] (MPa) (MPa) (MPa)
TRIP780 25.0 22.2 449 458 480 799 998 199
DP980 11.2 10.6 763 856 922 1027 1141 114
TABLE 49
Volume Percent Magnetic Phases (Fe
%) Before And After Tensile Testing
Difference Between
Magnetic Phases Magnetic Phases Magnetic Phases
Volume Before Volume After Volume Before and
Steel Tensile Test Tensile Test After Tensile Test
Grade (Fe %) (Fe %) (Fe %)
TRIP780 69.5 69.5 0.0
DP980 87.5 87.6 0.1
Incremental test data for each steel grade is listed in Table 50 and Table 51 and illustrated in FIG. 59 and FIG. 60.
TABLE 50
Incremental Test Data For TRIP 780 Steel
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) [MPa] [MPa] [MPa] (Fe %)
1 4.53 444 451 473 69.5
2 8.92 718 725 736 —
3 13.28 818 829 837 —
4 17.6 876 890 896 —
5 21.88 914 931 937 —
6 26.56 988 1007 1009 69.5
Change 544 556 536 0.0
(Last Cycle − First Cycle)
TABLE 51
Incremental Test Data For DP980 Steel
Magnetic
Cumulative 0.2% 0.5% 1.0% Phases
Applied Proof Proof Proof Volume
Cycle Strain Stress Stress Stress Percent
Number (%) [MPa] [MPa] [MPa] (Fe %)
1 4.32 749 844 911 87.5
2 8.65 1047 1067 1069 —
3 12.02 1115 1124 1110 87.6
Change 366 280 199 0.1
(Last Cycle − First Cycle)
This Case Example demonstrates less degree of strain hardening in commercial steel grades during deformation with no changes in magnetic phases volume percent (0 to 0.1 Fe % difference before and after deformation).
