Classifications

• • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • 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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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
• • 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

Definitions

• This present disclosure is directed at a new type of warm formable advanced high strength steel (AHSS).
• AHSS warm formable advanced high strength steel
• This steel can be warm formed due to its unique structure which allows it to develop relatively high strength without the need for austenitizing and quenching.
• the steel is then deformed to produce a part which can be a wide variety of structural and non- structural components. After deformation, the part is held to ensure the shape is maintained and then quenched in oil or water depending on the thickness of the part formed and the specific hardenability of the steel alloy. Often small additions of boron typically up to 0.05 wt are used to increase the hardenability of the steel which means that it opens up the process window for martensite formation. Upon proper quenching, the steel part then forms a martensitic structure which is strong and brittle. Subsequent heat treating is commonly done to produce tempered martensite which results in an improvement of ductility through sacrificing some of the strength levels. Summary
• the present disclosure is directed at steel alloys which may be wormed formed (treated at temperatures of 200 °C to 850 °C for time period of 1.0 second to 1 hour either by direct heating or induction heating).
• the elemental composition ranges (atomic percent) include: Fe present at 48.0 to 81.0, B at 2.0 - 8.0, Si at 4.0 to 14.0 and at least one austenite stabilizer (element that stabilizes austenite formation) comprising one or more of Cu, Mn and Ni, where the Cu is present at 0.1-6.0 atomic percent, Mn is present at 0.1 - 21.0 atomic percent and Ni is present at 0.1-16.0 atomic percent.
• one may include Cr at a level of up to 32.0 atomic percent.
• alloys herein that are suitable for warm forming include the Class 1 , Class 2 and Class 3 Steels described herein. Steel alloys of the present disclosure with application to centrifugal casting provide unique property combinations in wide ranges of strength and ductility depending on the aforementioned class of steel due to new enabling structure types facilitated by new enabling mechanisms.
• FIG. 1 Binary phase diagram for the iron rich region of the iron carbon binary system.
• FIG. 2 Binary Fe-C phase diagram illustrating the differences between new grades of warm forming steel (top call-out) and conventional steels (bottom call-out).
• FIG. 3 Model phase diagram indicating the expected phase equilibria of the new
• FIG. 4 illustrates structures and mechanisms regarding the formation of Class 1 Steel herein.
• FIG. 5 illustrates a representative stress-strain curve of a material with Modal Structure.
• FIG. 6 illustrates structures and mechanism regarding the formation of Class 2 steel alloys herein.
• FIG. 7 illustrates a stress-strain curve for the indicated structures and associated mechanisms in Class 2 alloys.
• FIG. 8 illustrates structures and mechanism regarding the formation of Class 3 steel alloys herein.
• FIG. 9 illustrates a stress-strain curve for the indicated structures and associated mechanisms in Class 3 alloys.
• FIG. 10 Picture of the plate in as-cast state.
• FIG. 11 NanoSteel sized R&D specimen geometry that was modified to increase the grip sections to 9.5 mm in order to accommodate 1/8" grip pinholes.
• FIG. 12 Temperature dependence of yield stress and tensile elongation in Alloy 213.
• FIG. 13 View of the Class 3 Alloy 36 specimen after HIP cycle and heat treatment before and after deformation to 57.5%.
• FIG. 14 Tensile strength, yield stress and tensile elongation as a function of testing temperature in commercial sheet from Alloy 82.
• the new class of warm forming steel does not need to be austenitized due to a much different metallurgy and enabling metallurgical transformations (i.e. not austenite to martensite).
• FIG. 1 the iron rich binary portion of the binary Fe-C phase diagram is shown. This diagram is used to describe the basic phase equilibria in -30,000 known worldwide equivalent iron and steel alloys.
• FIG. 2 the Fe-C binary phase diagram is utilized to show the differences between the new class of warm forming steels and conventional steels. Almost all conventional steels with the exception of austenitic stainless and TWIP (Twinning Induced Plasticity) steels are developed with main focus of heat treatment and structural development based on the eutectoid transformation.
• TWIP winning Induced Plasticity
• the first step is to heat the steel up to the single phase austenite region. Heating rate to the targeted temperature and time at temperature is important as the hardenability of the steel is sensitive to the average grain size of the material. Depending on how the steel is cooled or quenched from the austenitizing temperature will result in a wide range of characteristic structures produced including pearlite, upper and lower bainite, spherodite, and martensite. Additionally, complex or dual phase microstructures can be produced with different fractions of all of these characteristic microstructures along with ferrite, retained austenite, and cementite phases. As shown in FIG.
• the new class of warm forming steels is intrinsically different as the focus on phase and structural development is on the peritectic region and not the eutectoid region.
• the peritectic invariant reaction involves liquid with the specific transformation liquid + delta ferrite producing austenite. This is much different than the solid state eutectoid transformation which involves austenite producing ferrite plus cementite.
• FIG. 3 a model phase diagram for the warm forming alloys is provided in FIG. 3.
• the x-axis (labeled as Atomic Percent Alloying) is reference to an alloy that, as noted above, comprises Fe, B and Si, and at least one of Cu, Mn or Ni.
• the temperature on the y-axis will then vary depending upon the alloy selected.
• Transitions include the initial solidification through the peritectic transformation and the high temperature portion of the austenite to ferrite transformation associated with the gamma / austenite stability loop.
• the new type of steel produced herein may include any of the Class 1, Class 2 or
• Class 3 Steel Alloys noted herein that are warm formed, but preferably include warm forming of the Class 2 or Class 3 Steel Alloys.
• These Class 1, Class 2 and Class 3 Steel structure is stable to high temperatures and could be hot formed at conventional temperatures known for hot forming processes with typical hot forming ductility from 30 to 120%.
• the Class 1, Class 2 and Class 3 Steels herein exhibit relatively high strength and ductility at room temperature and maintains its high ductility at warm temperatures (i.e. 200 to 850°). Thus, it is applicable for cold deformation through a variety of methods including cold rolling, stamping, roll forming, hydroforming etc.
• the Class 1, Class 2 and Class 3 steel can now be treated by a warm forming process.
• warm forming the aforementioned steels are now heated up to a temperature range which is less than hot forming, typically 200 to 850°C, and for a time period of 1.0 seconds to 1 hour via direct heating (e.g. furnace heating) and/or induction heating.
• This temperature range is enabling for manufacturing for a number of key factors which will be described subsequently.
• warm forming may now reduce cost while producing new functionality through minimizing or avoiding springback issues found in cold forming steels. Enabling Advantages / New Functionality of Warm Forming Steels
• a cost factor limiting hot forming is the scale / oxide removal which forms during the elevated temperature exposure and then needs to be removed through existing shot / grit blasting processes.
• the oxidation occurs due to the elevated temperature exposure necessary to austenitize existing materials.
• the process does not lend itself to inert gas atmospheres because after hot forming, the parts must be quenched in a liquid medium to form martensite, thus creating additional oxidation.
• the temperature of deformation will be much lower which limits / prevents the oxidation typical for high temperature exposure.
• the Warm Forming steels do not need to be quenched and they exhibit an insensitive response to cooling rates in the solid state, the warm formed parts may be able to be processed while remaining in an inert atmosphere to prevent or minimize oxidation. This then is expected to result in a part which does not need to go through the expensive grit / shot blasting processes since scale formation is avoided.
• NanoModal Warm Forming Steels does not need to be water quenched and do not need to be heated up to the high temperatures found in conventional austenitizing. Thus, strict dimensional control is possible due to the lack of quench distortion. This results in a lower scrap rate and reduced cost enabling the technology.
• the non-stainless steel alloys herein are such that they are capable of formation of what is described herein as Class 1 Steel, Class 2 Steel or Class 3 Steel which are preferably crystalline (non-glassy) with identifiable crystalline grain size morphology.
• Class 1 Steel, Class 2 Steel or Class 3 Steel which are preferably crystalline (non-glassy) with identifiable crystalline grain size morphology.
• the ability of the alloys to form Class 1, Class 2 or Class 3 Steels herein is described in detail herein. However, it is useful to first consider a description of the general features of Class 1, Class 2 and Class 3 Steels, which is now provided below.
• Class 1 Steel herein is illustrated in FIG. 4.
• a Modal structure is initially formed which modal structure is the result of starting with a liquid melt of the alloy and solidifying by cooling, which provides nucleation and growth of particular phases having particular grain sizes.
• Reference herein to modal may therefore be understood as a structure having at least two grain size distributions.
• Grain size herein may be understood as the size of a single crystal of a specific particular phase preferably identifiable by methods such as scanning electron microscopy or transmission electron microscopy.
• Structure #1 of the Class 1 Steel may be preferably achieved by processing through either laboratory scale procedures and/or through industrial scale methods such as powder atomization or alloy casting.
• the Modal Structure of Class 1 Steel will therefore initially indicate, when cooled from the melt, the following grain sizes: (1) matrix grain size of 500 nm to 20,000 nm containing austenite and/or ferrite; (2) boride grain size of 25 nm to 500 nm (i.e. non-metallic grains such as M 2 B where M is the metal and is covalently bonded to B).
• the boride grains may also preferably be "pinning" type phases which is reference to the feature that the matrix grains will effectively be stabilized by the pinning phases which resist coarsening at elevated temperature.
• metal boride grains have been identified as exhibiting the M 2 B stoichiometry but other stoichiometries are possible and may provide pinning including M 3 B, MB (M1B1), M 23 B 6 , and M 7 B 3 .
• the Modal Structure of Class 1 Steel may be subjected to thermomechanical deformation and/or heat treatment, resulting in some variation in properties, but the Modal Structure may be maintained.
• Dynamic Nanophase Precipitation itself may be understood as the formation of a further identifiable phase in the Class 1 Steel which is termed a precipitation phase with an associated grain size. That is, the result of such Dynamic Nanophase Precipitation is to form an alloy which still indicates identifiable matrix grain size of 500 nm to 20,000 nm, boride pinning grain size of 25 nm to 500 nm, along with the formation of precipitation grains which contain hexagonal phases and grains of 1.0 nm to 200 nm. As noted above, the grain sizes therefore do not coarsen when the alloy is stressed, but does lead to the development of the precipitation grains as noted.
• references to the hexagonal phases may be understood as a dihexagonal pyramidal class hexagonal phase with a P6 3 i c space group (#186) and/or a ditrigonal dipyramidal class with a hexagonal P6bar2C space group (#190).
• the mechanical properties of such second type structure of the Class 1 Steel are such that the tensile strength is observed to fall in the range of 700 MPa to 1400 MPa, with an elongation of 10-50%.
• the second type structure of the Class 1 Steel is such that it exhibits a strain hardening coefficient from 0.1 to 0.4 that is nearly flat after undergoing the indicated yield.
• the value of the strain hardening exponent n lies between 0 and 1.
• a value of 0 means that the alloy is a perfectly plastic solid (i.e. the material undergoes non-reversible changes to applied force), while a value of 1 represents a 100% elastic solid (i.e. the material undergoes reversible changes to an applied force).
• Table 1A provides a comparison and performance summary for Class 1 Steel herein.
• Non metallic e.g. metal boride
• Non-metallic e.g. metal boride
• Strain Hardening Exhibits a strain hardening coefficient between 0.1 to 0.4 and a strain hardening Response
• Class 2 steel may also be formed herein from the identified alloys, which involves two new structure types after starting with Structure type #1, Modal Structure, followed by two new mechanisms identified herein as Static Nanophase Refinement and Dynamic Nanophase Strengthening.
• the new structure types for Class 2 Steel are described herein as Nanomodal Structure and High Strength Nanomodal Structure. Accordingly, Class 2 Steel herein may be characterized as follows: Structure #1 - Modal Structure (Step #1), Mechanism #1 - Static Nanophase Refinement (Step #2), Structure #2 - Nanomodal Structure (Step #3), Mechanism #2 - Dynamic Nanophase Strengthening (Step #4), and Structure #3 - High Strength Nanomodal Structure (Step #5).
• Structure #1 is initially formed in which Modal Structure is the result of starting with a liquid melt of the alloy and solidifying by cooling, which provides nucleation and growth of particular phases having particular grain sizes.
• Grain size herein may again be understood as the size of a single crystal of a specific particular phase preferably identifiable by methods such as scanning electron microscopy or transmission electron microscopy.
• Structure #1 of the Class 2 Steel may be preferably achieved by processing through either laboratory scale procedures and/or through industrial scale methods such as powder atomization or alloy casting.
• the Modal Structure of Class 2 Steel will therefore initially indicate, when cooled from the melt, the following grain sizes: (1) matrix grain size of 500 nm to 20,000 nm containing austenite and/or ferrite; (2) boride grain size of 25 nm to 500 nm (i.e. non-metallic grains such as M 2 B where M is the metal and is covalently bonded to B).
• the boride grains may also preferably be "pinning" type phases which are referenced to the feature that the matrix grains will effectively be stabilized by the pinning phases which resist coarsening at elevated temperature.
• metal boride grains have been identified as exhibiting the M 2 B stoichiometry but other stoichiometries are possible and may provide pinning including M 3 B, MB (M]Bi), M 23 B 6 , and M 7 B 3 and which are unaffected by Mechanisms #1 or #2 noted above).
• Reference to grain size is again to be understood as the size of a single crystal of a specific particular phase preferably identifiable by methods such as scanning electron microscopy or transmission electron microscopy.
• Structure #1 of Class 2 steel herein includes austenite and/or ferrite along with such boride phases.
• a stress strain curve is shown that represents the non-stainless steel alloys herein which undergo a deformation behavior of Class 2 steel.
• the Modal Structure is preferably first created (Structure #1) and then after the creation, the Modal Structure may now be uniquely refined through Mechanism #1, which is a Static Nanophase Refinement mechanism, leading to Structure #2.
• Static Nanophase Refinement is reference to the feature that the matrix grain sizes of Structure 1 which initially fall in the range of 500 nm to 20,000 nm are reduced in size to provide Structure 2 which has matrix grain sizes that typically fall in the range of 100 nm to 2000 nm.
• the boride pinning phase can change size significantly in some alloys, while it is designed to resist matrix grain coarsening during the heat treatments.
• the micron scale austenite phase (gamma-Fe) which was noted as falling in the range of 500 nm to 20,000 nm is partially or completely transformed into new phases (e.g. ferrite or alpha-Fe).
• the volume fraction of ferrite (alpha-iron) initially present in the Modal Structure (Structure 1) of Class 2 steel is 0 to 45%.
• the volume fraction of ferrite (alpha-iron) in Structure #2 as a result of Static Nanophase Refinement Mechanism #2 is typically from 20 to 80%.
• the static transformation preferably occurs during elevated temperature heat treatment and thus involves a unique refinement mechanism since grain coarsening rather than grain refinement is the conventional material response at elevated temperature.
• Structure #2 is uniquely able to transform to Structure #3 during Dynamic Nanophase Strengthening and as a result Structure #3 is formed and indicates tensile strength values in the range from 800 to 1800 MPa with 5 to 40% total elongation.
• nano-scale precipitates can form during Static Nanophase Refinement and the subsequent thermal process in some of the non-stainless high- strength steels.
• the nano-precipitates are in the range of 1 nm to 200 nm, with the majority (>50%) of these phases 10 ⁇ 20 nm in size, which are much smaller than the boride pinning phase formed in Structure #1 for retarding matrix grain coarsening.
• the boride grain sizes grow larger to a range from 200 to 2500 nm in size.
• the strength continues to increase but with a gradual decrease in strain hardening coefficient value up to nearly failure.
• Some strain softening occurs but only near the breaking point which may be due to reductions in localized cross sectional area at necking.
• the strengthening transformation that occurs at the material straining under the stress generally defines Mechanism #2 as a dynamic process, leading to Structure #3.
• dynamic it is meant that the process may occur through the application of a stress which exceeds the yield point of the material.
• the tensile properties that can be achieved for alloys that achieve Structure 3 include tensile strength values in the range from 800 to 1800 MPa and 5 to 40% total elongation. The level of tensile properties achieved is also dependent on the amount of transformation occurring as the strain increases corresponding to the characteristic stress strain curve for a Class 2 steel.
• tunable yield strength may also now be developed in Class 2 Steel herein depending on the level of deformation and in Structure #3 the yield strength can ultimately vary from 400 MPa to 1700 MPa. That is, conventional steels outside the scope of the alloys here exhibit only relatively low levels of strain hardening, thus their yield strengths can be varied only over small ranges (e.g., 100 to 200 MPa) depending on the prior deformation history. In Class 2 steels herein, the yield strength can be varied over a wide range (e.g. 400 to 1700 MPa) as applied to Structure #2 transformation into Structure #3, allowing tunable variations to enable both the designer and end users in a variety of applications, and utilize Structure #3 in various applications such as crash management in automobile body structures.
• a wide range e.g. 400 to 1700 MPa
• Structure #3 may be understood as a microstructure having matrix grains sized generally from 100 nm to 2000 nm which are pinned by boride phases which are in the range of 200 to 2500 nm and with precipitate phases which are in the range of 1 nm to 200 nm.
• the initial formation of the above referenced precipitation phase with grain sizes of 1 nm to 200 nm starts at Static Nanophase Refinement and continues during Dynamic Nanophase Strengthening leading to Structure 3 formation.
• the volume fraction of the precipitation phase with grain size from 1 nm to 200 nm in Structure 2 increases in Structure 3 and assists with the identified strengthening mechanism.
• the level of gamma-iron is optional and may be eliminated depending on the specific alloy chemistry and austenite stability.
• dynamic recrystallization is a known process but differs from Mechanism #2 (FIG. 6) since it involves the formation of large grains from small grains so that it is not a refinement mechanism but a coarsening mechanism. Additionally, as new undeformed grains are replaced by deformed grains no phase changes occur in contrast to the mechanisms presented here and this also results in a corresponding reduction in strength in contrast to the strengthening mechanism here. Note also that metastable austenite in steels is known to transform to martensite under mechanical stress but, preferably, no evidence for martensite or body centered tetragonal iron phases are found in the new steel alloys described in this application.
• Table IB below provides a comparison of the structure and performance features of Class 2 Steel herein. Table IB Comparison Of Structure and Performance of Class 2 Steel
• Hardening strain softening at initial vary from 0.2 to 1.0 depending straining as a result of phase on amount of deformation and Response
• Class 3 steel is associated with formation of a High Strength Lamellae Nanomodal Structure through a multi-step process as now described herein.
• Step #1 a preferred seven-step process is now disclosed and shown in FIG.8.
• Structure development starts from the Structure #1 - Modal Structure (Step #1).
• Mechanism #1 in Class 3 steel is now related to Lath Phase Creation (Step #2) that leads to Structure #2 - Modal Lath Phase Structure (Step #3), which through Mechanism #2 - Lamellae Nanophase Creation (Step #4) transforms into Structure #3 - Lamellae Nanomodal Structure (Step #5).
• Step #3 results in activation of Mechanism #3 - Dynamic Nanophase Strengthening (Step #6) which leads to formation of Structure #4 - High Strength Lamellae Nanomodal Structure (Step #7).
• Table 1C Reference is also made to Table 1C below.
• Modal Structure #1 involving the formation of the Modal Structures may be achieved in the alloys with the referenced chemistries in this application by processing through the laboratory scale as shown and/or through industrial scale methods involving chill surface processing such as twin roll casting or thin slab casting.
• the Modal Structure of Class 3 Steel will therefore initially indicate, when cooled from the melt, the following grain sizes: (1) matrix grain size of 500 nm to 20,000 nm containing ferrite or alpha-Fe (required) and optionally austenite or gamma-Fe; and (2) boride grain size of 100 nm to 2500 nm (i.e.
• non-metallic grains such as M 2 B where M is the metal and is covalently bonded to B); (3) yield strengths of 350 to 1000 MPa; (4) tensile strengths of 400 to 1200 MPa; and total elongation of 0-3.0%. It will also indicate dendritic growth morphology of the matrix grains.
• the boride grains may also preferably be "pinning" type phases which is reference to the feature that the matrix grains will effectively be stabilized by the pinning phases which resist coarsening at elevated temperature.
• metal boride grains have been identified as exhibiting the M 2 B stoichiometry but other stoichiometries are possible and may provide pinning including M 3 B, MB (M1B1), M 23 B 6 , and M7B 3 and which are unaffected by Mechanism #1, #2 or #3 noted above).
• Reference to grain size is again to be understood as the size of a single crystal of a specific particular phase preferably identifiable by methods such as scanning electron microscopy or transmission electron microscopy. Accordingly, Structure #1 of Class 3 steel herein includes ferrite along with such boride phases.
• Lath phase structure may be generally understood as a structure composed from plate-shaped crystal grains.
• Reference to "dendritic morphology” may be understood as tree-like and reference to “plate shaped” may be understood as sheet like.
• Lath structure formation preferably occurs at elevated temperature (e.g.
• Structure #2 also contains alpha-Fe and gamma-Fe remains optional.
• a second phase of boride precipitates with a size typically from 100 to 1000 nm may be found distributed in the lath matrix as isolated particles.
• the second phase of boride precipitates may be understood as non-metallic grains of different stoichiometry (M 2 B, M 3 B, MB (M ⁇ ]), M 23 B 6 , and M 7 B 3 ) where M is the metal and is covalently bonded to Boron.
• M is the metal and is covalently bonded to Boron.
• These boride precipitates are distinguished from the boride grains in Structure # 1 with little or no change in size.
• Structure #3 (Lamellae Nanomodal Structure) involves the formation of the lamellae morphology as a result of static transformation of ferrite into one or several phases through Mechanism #2 identified as Lamellae Nanophase Creation.
• Static transformation is a decomposition of the parent phase into new phase or several new phases due to alloying elements distribution by diffusion during elevated temperature heat treatment, which may preferably occur in the temperature range from 700°C to 1200°C.
• Lamellae (or layered) structure is composed of alternating layers of two phases whereby individual lamellae exist within a colony connected in three dimensions.
• Lamellae Nanomodal Structure contains: (1) lamellas of 100 nm to 1000 nm wide with a thickness in the range of 100 nm to 10,000 nm and with a length of 0.1 to 5 microns; (2) boride grains of 100 nm to 2500 nm of different stoichiometry (M 2 B, M 3 B, MB (M]Bi), M 23 B 6 , and M 7 B 3 ) where M is the metal and is covalently bonded to Boron, (3) precipitation grains of 1 nm to 100 nm; (4) yield strength of 350 MPa to 1400 MPa.
• the Lamellae Nanomodal Structure continues to contain alpha-Fe and gamma- Fe remains optional.
• Lamellae Nanomodal Structure transforms into Structure #4 through Dynamic Nanophase Strengthening (Mechanism #3, exposure to mechanical stress) during plastic deformation (i.e. exceeding the yield stress for the material) displaying relatively high tensile strengths in the range of 1000 MPa to 2000 MPa.
• a stress - strain curve is shown that represents the alloys with Structure #3 herein which undergo a deformation behavior of Class 3 steel as compared to that of Class 2.
• Structure #3 upon application of stress, provides the indicated curve, resulting in Structure #4 of Class 3 steel.
• the strengthening during deformation is related to phase transformation that occurs as the material strains under stress and defines Mechanism #3 as a dynamic process.
• lamellae structure is preferably formed prior to deformation.
• the micron scale austenite phase is transformed into new phases with reductions in microstructural feature scales generally down to the nanoscale regime.
• Some fraction of austenite may initially form in some Class 3 alloys during casting and then may remain present in Structure #1 and Structure #2.
• new or additional phases are formed with nanograins typically in a range from 1 to 100 nm.
• the ferrite grains contain alternating layers with nanostructure composed from new phases formed during deformation. Depending on the specific chemistry and the stability of the austenite, some austenite may be additionally present. In contrast with layers in Structure #3 where each layer represents a single or just few grains, in Structure #4, a large number of nanograins of different phases are present as a result of Dynamic Nanophase Strengthening. Since nanoscale phase formation occurs during alloy deformation, it represents a stress induced transformation and defined as a dynamic process. Nanoscale phase precipitations during deformation are responsible for extensive strain hardening of the alloys.
• the dynamic transformation can occur partially or completely and results in the formation of a microstructure with novel nanoscale / near nanoscale phases specified as High Strength Lamellae Nanomodal Structure (Structure #4) that provides high strength in the material.
• Structure #4 can be formed with various levels of strengthening depending on specific chemistry and the amount of strengthening achieved by Mechanism #3.
• Table 1C provides a comparison of the structure and performance features of Class 3 Steel herein.
• Ferrite optionally Ferrite, optionally
• Ferrite optionally Ferrite, optionally
• Matrix Grain Size 500 to 20,000 nm 100 to 10,000 nm microns in length
• non-uniform grains 100 nm - 1000 nm in
• Boride Grain Size 100 to 2,500 nm 100 to 2,500 nm 100 to 2,500 nm 100 to 2,500 nm
• Yield Strength 350 to 1000 MPa 300 to 1400 MPa 350 to 1400 MPa 500 to 1800 MPa
• Tensile Strength 200 to 1200 MPa 350 to 1600 MPa 1000 to 2000 MPa
• Strain hardening Strain hardening a high strain hardening
• coefficient may vary coefficient may vary coefficient at initial
• melting occurs in one or multiple stages with initial melting from ⁇ 1000°C depending on alloy chemistry and final melting temperature might be up to ⁇ 1500°C. Variations in melting behavior reflect a complex phase formation at chill surface processing of the alloys depending on their chemistry.
• the density of the alloys varies from 7.2 g/cm 3 to 8.2 g/cm 3 .
• the mechanical characteristic values in the alloys from each Class will depend on alloy chemistry and processing / treatment condition.
• the ultimate tensile strength values may vary from 700 to 1500 MPa with tensile elongation from 5 to 40%.
• the yield stress is in a range from 400 to 1300 MPa.
• the ultimate tensile strength values may vary from 800 to 1800 MPa with tensile elongation from 5 to 40%.
• the yield stress is in a range from 400 to 1700 MPa.
• the ultimate tensile strength values may vary from 1000 to 2000 MPa with tensile elongation from 0.5 to 15%.
• the yield stress is in a range from 500 to 1800 MPa. Additional classes of steel are anticipated with possible yield strengths, tensile strengths, and elongation values outside of the limits listed above.
• the chemical composition of the alloys studied is shown in Table 2 which provides the preferred atomic ratios utilized. These chemistries have been studied by using material processing through sheet casting in a Pressure Vacuum Caster (PVC). Using high purity elements or ferroadditives and other readily commercially available constituents, 35 g alloy feedstocks of the targeted alloys were weighed out according to the atomic ratios provided in Table 2. The feedstock material was then placed into the copper hearth of an arc-melting system. The feedstock was arc-melted into an ingot using high purity argon as a shielding gas. The ingots were flipped several times and re-melted to ensure homogeneity.
• PVC Pressure Vacuum Caster
• the ingots were then cast in the form of a finger approximately 12 mm wide by 30 mm long and 8 mm thick.
• the resulting fingers were then placed in a PVC chamber, melted using RF induction and then ejected onto a copper die designed for casting 3 by 4 inches sheets with thickness of 1.8 mm.
• An example of the cast plate is shown in FIG. 10. Utilized die casting of the alloys relates to the melt solidification at relatively high cooling rate that can be correlated with metal solidification at different sheet production methods including but not limited to sheet solidification on chill surface at twin roll, thin strip, and thin slab casting.
• the atomic percent of Fe present may therefore be 48.0, 48.1, 48.2, 48.3, 48.4, 48.5,
• 51.8 51.9, 52.0, 52.1, 52.2, 52.3, 52.4, 52.5, 52.6, 52.7, 52.8, 52.9, 53.0, 53.1, 53.2, 53.3,
• the atomic percent of B may therefore be 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6 2.7, 2.8, 2.9
• the atomic percent of Si may therefore be 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9,
• the atomic percent of Cu may therefore be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9,
• the atomic ratio of Mn may therefore be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 1.0,
• the atomic ratio of Ni may therefore be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6 2.7, 2.8, 2.9 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5.0, 5.1, 5.2,
• the atomic ratio of Cr as an optional element, if present, may therefore be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3,
• 21.8 21.9, 22.0, 22.1, 22.2, 22.3, 22.4, 22.5, 22.6, 22.7, 22.8, 22.9, 23.0, 23.1, 23.2, 23.3, 23.4, 23.5, 23.6, 23.7, 23.8, 23.9, 24.0, 24.1, 24.2, 24.3, 24.4, 24.5, 24.6, 24.7, 24.8, 24.9,
• CP feedstocks for Alloy 82 representing Class 2 steel were weighed out according to the atomic ratio provided in Table 2.
• the feedstock material was then placed into the copper hearth of an arc-melting system.
• the feedstock was arc-melted into an ingot using high purity argon as a shielding gas.
• the ingots were flipped several times and re- melted to ensure homogeneity.
• the resulting ingots were then placed in a PVC chamber, melted using RF induction and then ejected onto a copper die designed for casting a 3 x 4 inches plates with thickness of 1.8 mm.
• Resultant plate from the Alloy 82 was subjected to a HIP cycle at 1150 °C using an American Isostatic Press Model 645 machine with a molybdenum furnace with furnace chamber size of 4 inch diameter by 5 inch height. The plates were heated at 10°C/min until the target temperature and were exposed to an isostatic pressure of 30 ksi for 1 hour. Heat treatment at 850 °C for 1 hour was applied after HIP cycle. Tensile specimens with a gage length of 12 mm and a width of 3 mm were cut from the treated plate.
• the tensile measurements were done with testing parameters listed in Table 3 at temperatures specified in Table 4.
• the NanoSteel R&D specimen geometry (shown in FIG. 11) was modified by enlarging the grip section to accommodate for pinholes required for elevated temperature tensile testing.
• the modified grip section of the sample is 9.5mm (3/8") ⁇
• Table 5 a summary of the tensile test results including total tensile elongation (strain), yield stress, and ultimate tensile strength are shown for the treated plate from Alloy 82. Room temperature tensile property ranges for the same alloy after the same treatments are listed for comparison.
• ductility in high strength alloy is twice higher at 700 °C and reaches up to 92 % when tested at 800°C demonstrating high warm forming ability of the alloy.
• Warm temperature ductility of the alloys strongly depends on alloy chemistry, thermal mechanical treatment parameters and testing temperature. Table 3 Tensile Testing Parameters
• CP feedstocks for Alloy 213 representing Class 2 steel were weighed out according to the atomic ratio provided in Table 2.
• the feedstock material was then placed into the copper hearth of an arc-melting system.
• the feedstock was arc-melted into an ingot using high purity argon as a shielding gas.
• the ingots were flipped several times and re- melted to ensure homogeneity.
• the resulting ingots were then placed in a PVC chamber, melted using RF induction and then ejected onto a copper die designed for casting a 3 x 4 inches plates with thickness of 1.8 mm.
• Resultant plate from the Alloy 213 was subjected to a HIP cycle at 1125 °C using an
• the tensile measurements were done with testing parameters listed in Table 6 at temperatures specified in Table 7.
• Table 8 a summary of the tensile test results including total tensile elongation (strain), yield stress, and ultimate tensile strength are shown for the treated plate from Alloy 213.
• Room temperature tensile property ranges for the same alloy after the same treatments are listed for comparison. As can be seen, this alloy shows high ductility up to 74 % when tested at 700°C demonstrating high warm forming ability. Temperature dependence of yield stress and tensile elongation is illustrated on FIG. 12. Warm temperature ductility of the alloys strongly depends on alloy chemistry, thermal mechanical treatment parameters and testing temperature.
• Resultant plate from the Alloy 36 was subjected to a HIP cycle at 1100 °C using an American Isostatic Press Model 645 machine with a molybdenum furnace with furnace chamber size of 4 inch diameter by 5 inch height. The plates were heated at 10°C/min until the target temperature and were exposed to an isostatic pressure of 30 ksi for 1 hour. Heat treatment at 850 °C for 1 hour was applied after HIP cycle. Tensile specimens with NanoSteel R&D specimen geometry (FIG. 11) were cut from the treated plate.
• Alloy 82 was utilized for commercial sheet production by Thin Strip casting with inline hot rolling that was done at -1050 °C to -9% reduction. The condition of the sheet material is not optimized (partial transformation into NanoModal structure due to low temperature and reduction at in-line rolling).
• Tensile specimens with NanoSteel R&D specimen geometry (FIG. 11) were cut from the produced sheet. The tensile measurements were done with testing parameters listed in Table 10 at temperatures specified in Table 11.
• Table 12 a summary of the tensile test results including total tensile elongation (strain), yield stress, and ultimate tensile strength are shown for the produced sheet from Alloy 82. Temperature dependence of strength characteristics and tensile elongation is shown in FIG. 14. As it can be seen, that despite only partial transformation into NanoModal structure at inline hot rolling, the ductility of up to 30% can be achieved at 700 °C. Even higher warm forming ability is expected in the sheet with full transformation.

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