Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
  • B21D22/02—Stamping using rigid devices or tools
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
  • C21D8/0436—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
  • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets

Definitions

• 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.
• 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.
• 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.
• a method to develop yield strength distributions in a formed metal part comprising:
• FIG.s 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 micro structure in specimens cut from the stamped
• 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.
• 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.
• the starting condition is to supply a metal alloy.
• This metal alloy will comprise at least 70 atomic % iron.
• 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.
• 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.
• 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.
• 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.
• cold rolled thickness would be 0.5 to 10 mm thick.
• the cold rolled material is annealed to restore the ductility lost from the cold rolling process either partially or completely.
• sheet material from the alloys herein have a yield strength of Al (250 MPa to 750 MPa), a tensile strength of Bl (700 MPa to 1750 MPa), a true ultimate tensile strength of Cl (1100 MPa to 2300 MPa), and exhibits a total elongation Dl (10 % to 80%).
• a yield strength of Al 250 MPa to 750 MPa
• Bl 700 MPa to 1750 MPa
• Cl true ultimate tensile strength
• C 1100 MPa to 2300 MPa
• a total elongation Dl 10 % to 80%.
• 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 (Cl) is related to ultimate tensile strength (B 1) and can be calculated from the test data for each alloy herein using Eq.l. 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:
• 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.
• Step 3 in FIG. 2 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 l0°/s to l0 2 /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 l50°C and typically less than lOO°C.
• 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 l0°/s to l0 2 /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 micro structure 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.
• 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.
• this ferrite phase forms, it continues to deform through a dislocation mechanism contributing to sheet ductility and formability.
• 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.
• 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.
• 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.
• 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.
• 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.
• yield strength A2 MPa
• A3 MPa
• A4 MPa
• Distribution (iii) represents a maximum level of strengthening in the formed part with yield strength A4 in the range from 850 to 2300 MPa.
• 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.
• 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).
• the preferred levels of the elements 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), Al (0.4 to 5.2), Si (0.7 to 6.3), Cu (0.2 to 2.7), and C (0.3 to 3.7).
• 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 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.
• 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%).
• 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 l300°C depending on alloy chemistry, at a rate of 40°C/min. Temperature was then increased at lO°C/min to a max temperature between l425°C and l5l0°C depending on alloy chemistry. Once this maximum temperature was achieved, the sample was cooled at a rate of lO°C/min back to the initial ramp temperature before being reheated at lO°C/min to the maximum temperature.
• DSC Differential Scanning Calorimetry
• 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/cm 3 .
• the accuracy of this technique is ⁇ 0.01 g/cm 3 .
• 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 H00°C and l250°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 l000°C, depending on furnace temperature and final thickness.
• 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%.
• 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%.
• a Stress - strain curve example is provided showing the definition of 0.2%, 0.5% and 1.0% proof stresses .
• the 0.5% proof stress, or yield strength of the sheet (Al) 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.
• 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 -KFVs to l0 3 /s for the initial loading and after initial loading strain rates ranged from ⁇ l0 3 /s to ⁇ l0 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.
• 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 (Al) varies from 287 to 668 MPa and true ultimate tensile strength is in a range from 1175 to 2059 MPa.
• 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.
• 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%.
• Alloy 1 that indicates in the sheet an initial magnetic phase volume of 0.7 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%.
• the properties including yield change as a function of applied strain in sheet form.
• 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.
• 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 pm thickness was done by polishing with 9 pm, 3 pm, and 1 pm 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).
• PIPS Gatan Precision Ion Polishing System
• 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.
• 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 micro structure in the Alloy 7 sheet before deformation is shown by SEM and TEM micrographs in FIG. 38a and b, respectively.
• the microstructure consists primarily of recrystallized micron-sized austenite grains, 1 to 10 pm 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).
• 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).
• 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.
• 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.
• 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.
• phase transformation e.g. austenite to ferrite
• nanoscale phase formation e.g. creation of nanoferrite from 20 nm to 750 nm
• 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.
• FIG. 43a 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.
• FIG. 43d a HREM image of the nanoprecipitate example is shown.
• 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.
• 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.
• 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 5 th 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.
• FIG. 45 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.
• This Case Example demonstrates significant changes in magnetic phases volume percent in the stamping as compared to initial sheet. These changes correspond to micro structural transformation the unique NR&S mechanisms leading to sheet material strengthening as it deforms.
• 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
• 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
• 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.
• 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. 5 la for strength characteristics and in FIG. 5 lb 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.
• the current correlations based on 213 data points and shown in FIG. 5 la 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 FIG. 52a and b.
• 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.
• 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.
• 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.
• 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 pm thickness was done by polishing with 9 pm, 3 pm, and 1 pm 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).
• PIPS Gatan Precision Ion Polishing System
• 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.
• 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).
• FIG. 53a, c, and e the micro structure corresponding to that in the as stamped part is shown at the three levels of deformation.
• 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.
• 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 micro structure 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 micro structural 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 micro structural changes are consistent with the activation of the identified NR&S mechanism and conclusively show the material strengthening occurring in the stamping.
• 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.
• 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%.
• 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.
• 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.
• Incremental test data for each steel grade is listed in Table 50 and Table 51 and illustrated in FIG. 59 and FIG. 60.
• 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).

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