Patent Grant
10233524
This disclosure deals with steel alloys containing mixed microconstituent structure that has the ability to provide ductility at tensile strength levels at or above 900 MPa.
Steel has been used by mankind for at least 3,000 years and are widely utilized in industry comprising over 80% by weight of all metallic alloys in industrial use. Existing steel technology is based on manipulating the eutectoid transformation. The first step is to heat up the alloy into the single phase region (austenite) and then cool or quench the steel at various cooling rates to form multiphase structures which are often combinations of ferrite, austenite, and cementite. Depending on how the steel is cooled, a wide variety of characteristic microstructures (i.e. pearlite, bainite, and martensite) can be obtained with a wide range of properties. This manipulation of the eutectoid transformation has resulted in the wide variety of steels available currently.
Currently, there are over 25,000 worldwide equivalents in 51 different ferrous alloy metal groups. For steel, which is produced in sheet form, broad classifications may be employed based on tensile strength characteristics. Low Strength Steels (LSS) may be defined as exhibiting tensile strengths less than 270 MPa and include such types as interstitial free and mild steels. High-Strength Steels (HSS) may be defined as exhibiting tensile strengths from 270 to 700 MPa and include such types as high strength low alloy, high strength interstitial free and bake hardenable steels. Advanced High-Strength Steels (AHSS) steels may be defined as exhibiting tensile strengths greater than 700 MPa and include such types as martensitic steels (MS), dual phase (DP) steels, transformation induced plasticity (TRIP) steels, and complex phase (CP) steels. As the strength level increases, the ductility of the steel generally decreases. For example, LSS, HSS and AHSS may indicate tensile elongations at levels of 25% to 55%, 10% to 45% and 4% to 30%, respectively.
Steel material production in the United States is currently about 100 million tons per year and worth about $75 billion. According to the American Iron and Steel Institute, 24% of the US steel production is utilized in the auto industry. Total steel in the average 2010 vehicle was about 60%. New advanced high-strength steels (AHSS) account for 17% of the vehicle and this is expected to grow up to 300% by the year 2020. [American Iron and Steel Institute. (2013). Profile 2013. Washington, D.C.]
Continuous casting, also called strand casting, is one of the most commonly used casting process for steel production. It is the process whereby molten metal is solidified into a “semifinished” billet, bloom, or slab for subsequent rolling in the finishing mills (
The present disclosure is directed at a method for forming a mixed microconstituent steel alloy that begins with the method comprising: (a) supplying a metal alloy comprising Fe at a level of 61.0 to 81.0 atomic percent, Si at a level of 0.6 to 9.0 atomic percent, Mn at a level of 1.0 to 17.0 atomic percent; and B optionally up to 6.0 at. %; (b) melting the alloy and cooling and solidifying and forming an alloy that has a matrix grain size of 5.0 μm to 1000 μm and boride grains, if present, at a size of 1.0 μm to 50.0 μm; and (c) exposing the alloy formed in step (b) to heat and stress and forming an alloy that has matrix grains at a size of 1.0 μm to 100 μm, boride grains, if present, at a size of 0.2 μm to 10.0 μm and precipitation grains at a size of 1.0 nm to 200 nm.
The heat and stress in step (c) may comprise heating from 700° C. up to the solidus temperature of the alloy and wherein said alloy has a yield strength and said stress exceeds said yield strength. The stress may be in the range of 5 MPa to 1000 MPa. The alloy formed in step (c) may have a yield strength of 140 MPa to 815 MPa.
The alloy in step (c) may then be exposed to a mechanical stress to provide an alloy having a tensile strength of greater than or equal to 900 MPa and an elongation greater than 2.5%. More specifically, the alloy may have a tensile strength of 900 MPa to 1820 MPa and an elongation from 2.5% to 76.0%.
The alloy in step (c) may then be exposed to a mechanical stress to provide an alloy having matrix grain size of 100 nm to 50.0 μm and boride grain size of 0.2 μm to 10 μm. The alloy may also be characterized as having precipitation grains at a size of 1 nm to 200 nm. The alloy formed in step (c) may be further characterized as having mixed microconstituent structure comprising one group of matrix grains at a size of 0.5 μm to 50.0 μm and another group of matrix grains at a size of 100 nm to 2000 nm. The microconstituent group with matrix grain sizes from 0.5 μm to 50.0 μm contains primarily austenite matrix grains which may include a fraction of ferrite grains. The amount of austenite grains in this microconstituent group is from 50 to 100% by volume. The microconstituent group with 100 nm to 2000 nm matrix grains will contain primarily ferrite matrix grains which may include a fraction of austenite grains. The amount of ferrite grains in this microconstituent group is from 50 to 100% by volume. Note that the above amounts or ratios are only comparing ratios of matrix grains not including the boride, if present, or precipitate grains.
The alloy so formed in step (c) and exposed to mechanical stress may then be exposed to a temperature to recrystallize said alloy where said recrystallized alloy has matrix grains at a size of 1.0 μm to 50.0 μm. The recrystallized alloy will then indicate a yield strength and may be exposed to mechanical stress that exceeds said yield strength to provide an alloy having a tensile strength of at or greater than or equal to 900 MPa and an elongation of at or greater than 2.5%.
In related embodiment, the present disclosure is directed at an alloy comprising Fe at a level of 61.0 to 81.0 atomic percent, Si at a level of 0.6 to 9.0 atomic percent, Mn at a level of 1.0 to 17.0 atomic percent and B optionally up to 6.0 at. % characterized that the alloy contains mixed microconstituent structure comprising a first group of matrix grains of 0.5 μm to 50.0 μm, boride grains, if present, of 0.2 μm to 10.0 μm, and precipitation grains of 1.0 nm to 200 nm and a second group of matrix grains of 100 nm to 2000 nm, boride grains, if present, of 0.2 μm to 10.0 μm and precipitation grains of 1 nm to 200 nm. The alloy has a tensile strength of greater than or equal to 900 MPa and an elongation of greater than or equal to 2.5%. More specifically, the alloy has a tensile strength of 900 MPa to 1820 MPa and an elongation of 2.5% to 76.0%.
Accordingly, the alloys of present disclosure have application to continuous casting processes including belt casting, thin strip/twin roll casting, thin slab casting, thick slab casting, semi-solid metal casting, centrifugal casting, and mold/die casting. The alloys can be produced in the form of both flat and long products including sheet, plate, rod, rail, pipe, tube, wire and find particular application in a wide range of industries including but not limited to automotive, oil and gas, air transportation, aerospace, construction, mining, marine transportation, power, railroads.
The detailed description below may be better understood with reference to the accompanying FIGS. which are provided for illustrative purposes and are not to be considered as limiting any aspect of this invention.
The steel alloys herein have an ability for formation of a mixed microconstituent structure. The alloys therefore indicate relatively high ductility (e.g. elongations of greater than or equal to about 2.5%) at tensile strength levels at or above 900 MPa. Mixed microconstituent structure herein is characterized by a combination of structural features as described below and is represented by relatively coarse matrix grains with randomly distributed “pockets” of relatively more refined grain structure. The observed property combinations depend on the volume fraction of each structural microconstituent which is influenced by alloy chemistry and thermo-mechanical processing applied to the material.
The relatively high ductility steel alloys herein are such that they are capable of formation what is identified herein as a Mixed Microconstituent Structure. A schematic representation of such mixed structures is shown in
Mixed Microconstituent Structure formation including associated structures and mechanisms of formation are next shown in
The boride phase, if present, may also preferably be a “pinning” type, which is reference to the feature that the matrix grains will effectively be stabilized by the pinning phases with resistance to coarsening at elevated temperature. Note that the metal boride grains have been identified as exhibiting the M2B stoichiometry but other stoichiometry's are possible and may provide effective pinning including M3B, MB (M1B1), M23B6, and M7B3. Accordingly, Structure #1 of the High Ductility Steel alloys herein may be achieved by processing through either laboratory scale procedures and/or through industrial scale methods that include but not limited to thin strip casting, thin slab casting, thick slab casting, centrifugal casting, mold or die casting.
Deformation at elevated temperature (i.e. application of temperature and stress) of the High Ductility Steel alloys herein with initial Modal Structure leads to refinement and homogenization of the Modal Structure through Dynamic Nanophase Refinement (Mechanism #1,
The formation of the Homogenized Nanomodal Structure can occur in one or in several steps and may occur partially or completely. In practice, this may occur for instance during the normal hot rolling of slabs after initial casting. The slabs may be placed in a tunnel furnace and reheated and then roughing mill rolled which may be include multiple stands or in a reversing mill and then subsequently rolled to an intermediate gauge and then the hot slab can be further processed with or without additional reheating, finished to a final hot rolled gauge thickness in a finishing mill which may or may not be in multiple stages/stands. During each step of the rolling process, the Dynamic NanoPhase Refinement will occur until the Homogenized Nanomodal Structure is fully formed and the targeted gauge reduction is achieved.
Mechanical properties of the High Ductility Steel alloys with Homogenized Nanomodal Structure depend on alloy chemistry and their phase composition (volume fraction of High Strength Nanomodal Structure vs Modal Nanophase Structure) and will vary with a yield strength from about 140 to 815 MPa. Note that after stress is applied which exceeds the yield strength then the Homogenized Nanomodal Structure begins to transform to the Mixed Microconstituent Structure (Structure #3,
The Homogenized Nanomodal Structure will transform into a Mixed Microconstituent Structure (Structure #3,
In
Homogenized Nanomodal Structure (Structure #2,
During the deformation, Dynamic Nanophase Strengthening (Mechanism #2,
After plastically deforming, Dynamic Nanophase Strengthening (Mechanism #2,
When fully recrystallized, the Structure #2a contains few dislocations or twins, but stacking faults can be found in some recrystallized grains. Depending on the alloy chemistry and heat treatment, the equiaxed recrystallized austenite matrix grains can range from 1 μm to 50 μm in size while M2B boride phase is in the range of 0.2 μm to 10 μm with precipitate phases in the range from 1 nm to 200 nm. Mechanical properties of Recrystallized Modal Structure (Structure #2a,
The ability of the new High Ductility Steel alloys herein to form Homogenized/Recrystallized Modal Structure (Structure #2/2a,
The chemical composition of the alloys herein is shown in Table 4 which provides the preferred atomic ratios utilized. These chemistries have been used for material processing through slab casting in an Indutherm VTC800V vacuum tilt casting machine. Alloys of designated compositions were weighed out in 3 kilogram charges using designated quantities of commercially-available ferroadditive powders of known composition and impurity content, and additional alloying elements as needed, according to the atomic ratios provided in Table 4 for each alloy. Weighed out alloy charges were placed in zirconia coated silica-based crucibles and loaded into the casting machine. Melting took place under vacuum using a 14 kHz RF induction coil. Charges were heated until fully molten, with a period of time between 45 seconds and 60 seconds after the last point at which solid constituents were observed, in order to provide superheat and ensure melt homogeneity. Melts were then poured into a water-cooled copper die to form laboratory cast slabs of approximately 50 mm thick that is in the thickness range for Thin Slab Casting process (
From the above it can be seen that the alloys herein that are susceptible to the transformations illustrated in
From the above, one of skill in the art would understand the alloy composition herein to include the following three elements at the following indicated atomic percent: Fe (61-81 at. %); Si (0.6-9.0 at. %); Mn (1.0-17.0 at. %). In addition, it can be appreciated that the following elements are optional and may be present at the indicated atomic percent: Ni (0.1-13.0 at. %); Cr (0.1-12.0 at. %); B (0.1-6.0 at. %); Cu (0.1-4.0 at. %); C (0.1-4.0 at. %). Impurities may be present include Al, Mo, Nb, S, O, N, P, W, Co, Sn, Zr, Pd and V, which may be present up to 10 atomic percent.
Thermal analysis of the alloys herein was performed on the as-solidified cast slab samples on a Netzsch Pegasus 404 Differential Scanning calorimeter (DSC). Measurement profiles consisted of a rapid ramp up to 900° C., followed by a controlled ramp to 1425° C. at a rate of 10° C./minute, a controlled cooling from 1425° C. to 900° C. at a rate of 10° C./min, and a second heating to 1425° C. at a rate of 10° C./min. Measurements of solidus, liquidus, and peak temperatures were taken from the final heating stage, in order to ensure a representative measurement of the material in an equilibrium state with the best possible measurement contact. In the alloys listed in Table 4, melting occurs in one or multiple stages with initial melting from ˜1080° C. depending on alloy chemistry and final melting temperature exceeding 1450° C. in some cases (Table 5). Variations in melting behavior reflect a complex phase formation during solidification of the alloys depending on their chemistry.
The 50 mm thick laboratory slabs from each alloy were subjected to hot rolling at the temperature of 1075 to 1100° C. depending on alloy solidus temperature. Rolling was done on a Fenn Model 061 single stage rolling mill, employing an in-line Lucifer EHS3GT-B18 tunnel furnace. Material was held at the hot rolling temperature for an initial dwell time of 40 minutes to ensure homogeneous temperature. After each pass on the rolling mill, the sample was returned to the tunnel furnace with a 4 minute temperature recovery hold to partially adjust for temperature loss during each hot rolling pass. Hot rolling was conducted in two campaigns, with the first campaign achieving approximately 85% total reduction to a thickness of 6 mm. Following the first campaign of hot rolling, a section of sheet between 150 mm and 200 mm long was cut from the center of the hot rolled material. This cut section was then used for a second campaign of hot rolling for a total reduction between both campaigns of between 96% and 97%. A list of specific hot rolling parameters used for all alloys is available in Table 6. An example of the hot rolled sheet from Alloy 59 is shown in
The density of the alloys was measured on-sections of cast material that had been hot rolled to between 6 mm and 9.5 mm. Sections were cut to 25 mm×25 mm dimensions, and then surface ground to remove oxide from the hot rolling process. Measurements of bulk density were taken from these ground samples, 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 7 and was found to vary from 7.40 g/cm3 to 7.90 g/cm3. Experimental results have revealed that the accuracy of this technique is ±0.01 g/cm3.
The fully hot-rolled sheets from selected alloys were then subjected to further cold rolling in multiple passes. Rolling was done on a Fenn Model 061 single stage rolling mill. A list of specific cold rolling parameters used for the alloys is shown in Table 8. An example of the cold rolled sheet from Alloy 59 is shown in
After hot and cold rolling, tensile specimens and SEM samples were cut via EDM. The resultant samples were heat treated at the parameters specified in Table 9. Heat treatments were conducted in a Lucifer 7GT-K12 sealed box furnace under an argon gas purge, or in a ThermCraft XSL-3-0-24-1C tube furnace. In the case of air cooling, the specimens were held at the target temperature for a target period of time, removed from the furnace and cooled down in air. In cases of controlled cooling, the furnace temperature was lowered at a specified rate with samples loaded.
Tensile specimens were tested in the hot rolled, cold rolled, and heat treated conditions. 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 room temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
Tensile properties of the alloys in the as hot rolled condition are listed in Table 10. The ultimate tensile strength values may vary from 786 to 1524 MPa with tensile elongation from 17.4 to 63.4%. The yield stress is in a range from 142 to 812 MPa. Mechanical properties of the steel alloys herein depend on alloy chemistry, processing conditions, and material mechanistic response to the processing conditions.
Tensile properties of selected alloys after hot rolling and subsequent cold rolling are listed in Table 11.
The ultimate tensile strength values may vary from 1159 to 1707 MPa with tensile elongation from 2.6 to 36.4%. The yield stress is in a range from 796 to 1388 MPa. Mechanical properties of the steel alloys herein depend on alloy chemistry, processing conditions, and material mechanistic response to the processing conditions.
Tensile properties of the hot rolled sheets after hot rolling with subsequent heat treatment at different parameters (Table 9) are listed in Table 12. The ultimate tensile strength values may vary from 900 MPa to 1205 MPa with tensile elongation from 30.1 to 68.4%. The yield stress is in a range from 245 to 494 MPa. Mechanical properties of the steel alloys herein depend on alloy chemistry, processing conditions, and material mechanistic response to the processing conditions.
Tensile properties of the selected alloys after hot rolling with subsequent cold rolling and heat treatment at different parameters (Table 9) are listed in Table 13. The ultimate tensile strength values may vary from 901 MPa to 1493 MPa with tensile elongation from 30.0 to 76.0%. The yield stress is in a range from 217 to 657 MPa. As it can be seen, advanced property combinations with high and tensile strength above 900 MPa can be achieved in the sheet material from High Ductility Alloys herein after full post processing including hot rolling, cold rolling and heat treatment.
Tensile properties of selected alloys were compared with tensile properties of existing steel grades. The selected alloys and corresponding treatment parameters are listed in Table 14. Tensile stress-strain curves are compared to that of existing Dual Phase (DP) steels (
Using commercial purity feedstock, a 3 kg charge of selected alloys were weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slabs in an Indutherm VTC800V vacuum tilt casting machine. Tensile specimens were made from sections close to the bottom of cast slabs by electric discharge machine (EDM). Tensile properties of the alloys in the as cast condition are listed in 15. The ultimate tensile strength values may vary from 440 to 881 MPa with tensile elongation from 1.4 to 20.2%. The yield stress is in a range from 192 to 444 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry.
The microstructure of the Alloy 8 slab in as-cast state was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For SEM study, the cross-section of the cast slab was ground on SiC abrasive papers with reduced grit size, and then polished progressively with diamond media paste down to 1 μm. The final polishing was done with 0.02 μm grit SiO2 solution. Microstructures were examined by scanning electron microscopy (SEM) using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc. To prepare TEM specimens, the EDM cut piece was first thinned by grinding with pads of reduced grit size every time, and further thinned to 60 to 70 μm thickness by polishing with 9 μm, 3 μm and 1 μm diamond suspension solution, respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled with electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling was done at 4.5 Kev, and the inclination angle was reduced from 4° to 2° to open up the thin area. The TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV.
SEM backscattered images of Alloy 8 as-cast slab show a dendritic matrix phase with M2B boride phase at the grain boundaries, as shown in
This Case Example illustrates that a formation of Modal Structure (Structure #1,
Using commercial purity feedstock, a 3 kg charge of Alloy 8 was weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine that was then processed with a two-step hot rolling at 1075° C. by a rolling strain of 87.5% and 73.4%, respectively (total reduction is ˜97%). The thickness of hot rolled sheet was ˜1.7 mm. The tensile specimen was cut from the sheet material after hot rolling using wire electrical discharge machining (EDM). Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. The test was run at room temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture. Corresponding stress-strain curve is shown in
To make SEM specimens, the cross-section samples of the sheets were cut and ground by SiC paper and then polished progressively with diamond media paste down to 1 μm grit. The final polishing was done with 0.02 μm grit SiO2 solution. The microstructure at central layer region of cross-section of sheet was observed, imaged and evaluated. SEM microscopic analysis was conducted using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc. Microstructure of hot rolled samples studied by SEM is shown in
Additional details of the Alloy 8 structure were revealed using X-ray diffraction. X-ray diffraction was done using a Panalytical X'Pert MPD diffractometer with a Cu Kα x-ray tube and operated at 45 kV with a filament current of 40 mA. Scans were run with a step size of 0.01° and from 25° to 95° two-theta with silicon incorporated to adjust for instrument zero angle shift. The resulting scans were then subsequently analyzed using Rietveld analysis using Siroquant software. In
As can be seen in Table 16, after hot rolling (at 1075° C. with 97% reduction) three phases are found which are γ-Fe (austenite), M2B1 phase, and ditrigonal dipyramidal hexagonal phase. The presence of the hexagonal phase is a characteristic feature of Dynamic Nanophase Refinement (Mechanism #1,
To examine the structural features of the Alloy 8 structure in more detail, high resolution transmission electron microscopy (TEM) was utilized. To prepare TEM specimens, the gage sections of tensile tested samples were first cut with EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to 60 to 70 μm thickness was done by polishing with 9 μm, 3 μm, and 1 μm diamond suspension solution respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled with electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling was done at 4.5 Kev, and the inclination angle was reduced from 4° to 2° to open up the thin area. The TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV.
The TEM images of Alloy 8 microstructure after the hot rolling and tensile deformation are shown in
This Case Example illustrates a formation of the Mixed Microconstituent Structure through Dynamic Nanophase Strengthening in “pockets” of hot rolled Alloy 8 sample microstructure upon deformation when transformed microconstituent regions of High Strength Nanomodal Structure with refined grains and microconstituent regions of Modal Nanophase Structure.
The Alloy 8 hot rolled sheet from previous Case Example #3 was heat treated at 950° C. for 6 hr and at 1075° C. for 2 hr. The tensile specimens were cut from the sheet material after hot rolling and heat treatment using wire electrical discharge machining (EDM). Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. The tests were run at room temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture. Corresponding stress-strain curves are shown in
To make SEM specimens, the cross-section samples of the sheets were cut and ground by SiC paper and then polished progressively with diamond media paste down to 1 μm grit. The final polishing was done with 0.02 μm grit SiO2 solution. The microstructure at central layer region of cross-section of sheet was observed, imaged and evaluated. SEM microscopic analysis was conducted using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
Additional details of the Alloy 8 structure after hot rolling and heat treatment at 950° C. for 6 hours were revealed using X-ray diffraction. X-ray diffraction was done using a Panalytical X'Pert MPD diffractometer with a Cu Kα x-ray tube and operated at 45 kV with a filament current of 40 mA. Scans were run with a step size of 0.01° and from 25° to 95° two-theta with silicon incorporated to adjust for instrument zero angle shift. The resulting scans were then subsequently analyzed using Rietveld analysis using Siroquant software. In
As can be seen in Table 17, after hot rolling (at 1075° C. with 97% reduction) and heat treatment (950° C. for 6 hours), four phases were identified: γ-Fe (austenite), M2B1 phase, ditrigonal dipyramidal hexagonal phase and dihexagonal pyramidal hexagonal phase. As compared to phase composition of Alloy 8 after hot rolling only (Table 16), a second hexagonal phase is formed upon heat treatment suggesting phase transformation in addition to recrystallization. After tensile deformation, a fifth phase, α-Fe, was found in the sample, suggesting further austenite transformation under tensile stress. Along with additional phase formation, the lattice parameters of initial phases change indicating that the amount of solute elements dissolved in these phases have changed. This would indicate that phase transformations are induced by elements redistribution under the applied stress.
To examine the structural features of the Alloy 8 after hot rolling (at 1075° C. with 97% reduction) and heat treatment (950° C. for 6 hours) in more detail, high resolution transmission electron microscopy (TEM) was utilized. To prepare TEM specimens, the samples were first cut with EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to 60 to 70 μm thickness was done by polishing with 9 μm, 3 μm, and 1 μm diamond suspension solution respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled with electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens were ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling was done at 4.5 Kev, and the inclination angle was reduced from 4° to 2° to open up the thin area. The TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV.
The TEM images of hot rolled Alloy 8 slab sample after heat treatments at 950° C. and 1075° C. are shown in
This Case Example illustrates the formation of the Mixed Microconstituent Structure upon deformation of the alloy in hot rolled and heat treated state where transformed regions of High Strength Nanomodal Structure with refined grains are distributed in the Modal NanoPhase Structure of the un-transformed matrix.
Using commercial purity feedstock, a 3 kg charge of Alloy 8 was weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine that was then processed with a two-step hot rolling at 1075° C. by rolling strains of 87.5% and 73.4%. The final thickness of the hot rolled sheet was 1.7 mm. Hot rolled Alloy 8 sheet was further cold rolled by 19.2% to 1.4 mm thickness. Cold rolled Alloy 8 sheet was heat treated at 950° C. for 6 hr. Tensile specimens were cut from the sheet material after cold rolling and after cold rolling and heat treatment using wire electrical discharge machining (EDM). Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. The test was run at room temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture. Corresponding stress-strain curves are shown in
To make SEM specimens, the cross-section samples of the sheets were cut and ground by SiC paper and then polished progressively with diamond media paste down to 1 μm grit. The final polishing was done with 0.02 μm grit SiO2 solution. The microstructure at the central layer of cross-section of sheet was observed, imaged, and evaluated. SEM microscopic analysis was conducted using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
Additional details of the Alloy 8 structure are revealed using X-ray diffraction. X-ray diffraction was done using a Panalytical X'Pert MPD diffractometer with a Cu Kα x-ray tube and operated at 45 kV with a filament current of 40 mA. Scans were run with a step size of 0.01° and from 25° to 95° two-theta with silicon incorporated to adjust for instrument zero angle shift. The resulting scans were then subsequently analyzed using Rietveld analysis using Siroquant software. In
As can be seen in Table 18, four phases were identified: γ-Fe (austenite), α-Fe (ferrite), M2B1 phase, and ditrigonal dipyramidal hexagonal phase in all cases when cold rolling was applied. However, the lattice parameters of the phases change indicating that the amount of solute elements dissolved in these phases have changed depending on the alloy processing.
To examine the structural features of the Alloy 8 structure in more detail, high resolution transmission electron microscopy (TEM) was utilized. To prepare TEM specimens, the samples were first cut with EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to 60 to 70 μm thickness was done by polishing with 9 μm, 3 μm, and 1 μm diamond suspension solution respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled with electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens were ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling was done at 4.5 Kev, and the inclination angle was reduced from 4° to 2° to open up the thin area. The TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV.
The TEM images of Alloy 8 after cold rolling are shown in
After the cold-rolled sample was heat treated at 950° C. for 6 hrs, a recrystallized microstructure was observed to be formed. As shown in
This Case Example illustrates the formation of the Mixed Microconstituent Structure upon deformation of the alloy by cold rolling and after tensile deformation of cold rolled and heat treated Alloy 8 when transformed regions of High Strength Nanomodal Structure with refined grains are distributed in the Modal Nanophase Structure of the un-transformed matrix.
Using commercial purity feedstock, a 3 kg charge of Alloy 44 was weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine. The slab was then processed with a two-step hot rolling at 1100° C. by a rolling strain of 87.4% and 73.9%, respectively (total reduction is ˜97%). The thickness of hot rolled sheet was ˜1.7 mm. Hot rolled Alloy 44 sheet was further cold-rolled by 19.3% to ˜1.4 mm thickness. The tensile specimens were cut from the sheet material after hot rolling and after cold rolling using wire electrical discharge machining (EDM). Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. The test was run at room temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture. Tensile properties of the Alloy 44 after hot and cold rolling are shown in
Cold rolled and heat treated sheet was then cold rolled again with reduction of 22.3% with following heat treatment at 850° C. for 10 min. Measured tensile properties are shown in
This Case Example illustrates property recovery in the High Ductility Steel alloy through cycles of cold rolling and heat treatment. The process of Mixed Microconstituent Structure (Structure #3,
Using commercial purity feedstock, a 3 kg charge of Alloy 43 and Alloy 44 were weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine that was then processed with a two-step hot rolling with parameters specified in Table 6. The thickness of hot rolled sheet was ˜1.7 mm. Hot rolled sheet was further cold-rolled with reductions of 10, 20 and 30% for Alloy 43 and 7, 20, 26, and 43% for Alloy 44. The tensile specimens were cut from the sheet material after hot rolling and after cold rolling using wire electrical discharge machining (EDM). Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. The test was run at room temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
This Case Example illustrates that property combinations in the High Ductility Steel alloys can be controlled by the level of cold rolling reduction depending on the end user property requirements. The level of cold rolling reduction affects the volume fraction of the transformed High Strength Nanomodal Structure (Structure #3b,
Using commercial purity feedstock, a 3 kg charge of Alloy 8 and Alloy 44 were weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine that was then processed with a two-step hot rolling with corresponding parameters specified in Table 6. Hot rolled sheet from Alloy 44 was then subjected to further cold rolling in multiple passes, with a total reduction of approximately 25%. Rolling was done on a Fenn Model 061 single stage rolling mill. Specific cold rolling parameters used for the alloy is shown in Table 8. Cold rolled sheet from alloy 44 was annealed at 850° C. for 5 min. Tensile specimens were cut via EDM from hot rolled sheet of Alloy 8 and hot rolled, cold rolled and heat treated sheet of Alloy 44. The specimens were incrementally tested in tension. Tensile testing was performed on an Instron Model 3369 mechanical testing frame, using the Instron Bluehill control and analysis software. Samples were tested at room temperature under displacement control at a strain rate of 1×10−3 per second. Samples were mounted to a stationary bottom fixture, and a top fixture attached to a moving crosshead. A 50 kN load cell was attached to the top fixture to measure load. Each tensile test was run to a total tensile elongation of 4%, after which the samples were unloaded and re-measured, and then tested again. This process was continued until the sample failed during testing. The resultant stress-strain curves for Alloy 8 and Alloy 44 at incremental testing are shown in
Very high strength can be achieved in the High Ductility Steel alloys through cold rolling. As shown in
This Case Example illustrates hardening in the High Ductility Steel alloys through Dynamic Nanophase Strengthening with the Mixed Microconstituent Structure (Structure #3,
Using commercial purity feedstock, a 3 kg charge of Alloy 44 was weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine that was hot rolled to 2.5 mm, and subsequently cold rolled to 1.2 mm. Rolling was done on a Fenn Model 061 single stage rolling mill. Hot rolling used an in-line Lucifer EHS3GT-B18 tunnel furnace, with the rolled material heated to 1100° C., using an initial dwell time of 40 minutes to ensure homogeneous starting temperature, and a 4 minute temperature recovery hold in between each hot rolling pass. Cold rolling employed the same rolling mill, but without the use of the in-line tunnel furnace. Tensile specimens were cut from the cold rolled material via EDM, and then heat treated at 850° C. for 10 minutes with air cooling. Heat treatment was conducted in a Lucifer 7GT-K12 sealed box furnace under an argon gas purge. Heat treated specimens were ground on a belt sander to remove oxide from the specimen surface, and then tensile tested. Tensile testing was performed on Instron Model 3369 and Instron Model 5984 mechanical testing frames, using the Instron Bluehill control and analysis software. Samples were tested at room temperature under displacement control at a strain rates listed in Table 19. Samples were mounted to a stationary bottom fixture, and a top fixture attached to a moving crosshead. A load cell was attached to the top fixture to measure load. The load limit of the 3369 load cell was 50 kN, and the load limit for the 5984 load cell was 150 kN. In order to determine the actual strain rates observed by the samples, with a minimal influence of machine compliance, sample strain was measured using an advanced video extensometer (AVE). These measurements were plotted over time, and an approximate average rate of strain was calculated from the slope of a line fit to the resulting plot of values. Results of the tests are plotted as strain rate dependence of yield stress, ultimate tensile strength, strain hardening exponent, and tensile elongation shown in
This Case Example illustrates that strain rate does not affect yield stress of the material but influences material behavior after yielding when Dynamic Nanophase Strengthening (Mechanism #2,
Using commercial purity feedstock, 3 kg charges of Alloy 114, Alloy 115 and Alloy 116 were weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine. In the center of the cast plate was a shrinkage funnel that was created by the solidification of the last amount of molten metal liquid. A schematic illustration of the cross section through the center of the cast slab with the marked positions where the samples for chemical analysis were taken from is shown in
The results of the chemical analysis are shown in
This Case Example illustrates that High Ductility Steel alloys solidify uniformly and do not show any chemical macrosegregation through cast volume. This clearly indicates that the process window for production is much greater than the 50 mm used in this example and it is both feasible and anticipated to expect the mechanisms presented here-in to be active through the 20 to 500 mm as-cast thickness of the commercial continuous casting of the alloys presented here-in.
Using commercial purity feedstock, a 3 kg charge of Alloy 8 was weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine. Cast laboratory slabs were subjected to hot rolling using a Fenn Model 061 Rolling Mill and a Lucifer 7-R24 Atmosphere Controlled Box Furnace. The slabs were placed in a hot furnace pre-heated to 1100° C. and held for 40 minutes prior to the start of rolling. The plates were then hot rolled with multiple passes of 10% to 25% reduction mimicking multi-stand hot rolling at the Continuous Slab Casting processes (
To analyze the microstructure changes during hot rolling and after heat treatment, samples after casting, hot rolling and heat treatments were examined by the SEM. To make SEM specimens, the cross-sections of the sheet samples were cut and ground by SiC paper and then polished progressively with diamond media paste down to 1 μm grit. The final polishing was done with 0.02 μm grit SiO2 solution. Microstructures of sheet samples from Alloy 8 after hot rolling and heat treatment were examined by scanning electron microscopy (SEM) using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
This Case Example demonstrates an ability for as-cast microstructure of High Ductility Steel alloys to be homogenized by hot rolling with formation of uniform Homogenized NanoModal Structure (Structure #2,
Using commercial purity feedstock, a 3 kg charge of Alloy 20 was weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine. Cast laboratory slabs were subjected to hot rolling using a Fenn Model 061 rolling mill and a Lucifer 7-R24 atmosphere controlled box furnace. The slabs were placed in a hot furnace pre-heated to 1100° C. and held for 40 minutes prior to the start of rolling. The plates were then hot rolled with multiple passes of 10% to 25% reduction mimicking multi-stand hot rolling at the Continuous Slab Casting processes (
To analyze the microstructure changes during hot rolling and after heat treatment, samples after casting, hot rolling and heat treatment were examined by SEM. To make SEM specimens, the cross-sections of the sheet samples were cut and ground by SiC paper and then polished progressively with diamond media paste down to 1 μm grit. The final polishing was done with 0.02 μm grit SiO2 solution. Microstructures of sheet samples from Alloy 8 after hot rolling and heat treatment were examined by scanning electron microscopy (SEM) using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
This Case Example demonstrates an ability for as-cast microstructure of High Ductility Steel alloys to be homogenized by hot rolling with formation of uniform Homogenized NanoModal Structure (Structure #2,
Using commercial purity feedstock, Alloy 44 was cast, hot rolled at 1100° C. with subsequent cold rolling to final thickness of 1.2 mm. Rolling was done on a Fenn Model 061 single stage rolling mill. Hot rolling used an in-line Lucifer EHS3GT-B18 tunnel furnace, with the rolled material heated to 1075° C., using an initial dwell time of 40 minutes to ensure homogeneous temperature, and a 4 minute temperature recovery hold in between each hot rolling pass. Cold rolling employed the same rolling mill, but without the use of the in-line tunnel furnace. Two types of heat treatment were applied to cold rolled sheet: 850° C. for 6 hr imitating batch annealing of coils at commercial sheet production and at 850° C. for 10 min imitating in-line annealing of coils on continuous lines at commercial sheet production. Both heat treatments used a furnace temperature of 850° C. Heat treatments were conducted in a Lucifer 7GT-K12 sealed box furnace under an argon gas purge. Tensile specimens were cut via EDM and heat treated according to the treatments outlined in Table 20. Heat treated specimens were ground on a belt sander to remove oxide from the specimen surface, and then tensile tested. Tensile testing was performed on an Instron Model 3369 mechanical testing frame, using the Instron Bluehill control and analysis software. Samples were tested at room temperature under displacement control at a strain rate of 1×10−3 per second. Samples were mounted to a stationary bottom fixture, and a top fixture attached to a moving crosshead. A 50 kN load cell was attached to the top fixture to measure load.
Tensile properties of Alloy 44 after hot rolling, cold rolling and both types of annealing are shown in Table 20 and illustrated
This Case Example illustrates that properties of High Ductility Steel alloys might be controlled by heat treatment that can be applied to commercially produced sheet coils either by batch annealing or by annealing on a continuous line.
Elastic modulus was measured for selected alloys. Using commercial purity feedstock, 3 kg charge were weighed out according to the alloy stoichiometry in Table 4 and cast into 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine that was then processed with a two-step hot rolling with corresponding parameters specified in Table 6. Hot rolled sheets were then subjected to further cold rolling in multiple passes, with a total reduction of approximately 25%. Rolling was done on a Fenn Model 061 single stage rolling mill. A list of specific cold rolling parameters used for the alloys is shown in Table 7. All resultant sheets were heat treated in a Lucifer 7GT-K12 sealed box furnace under an argon gas purge at 1050° C. for 5 minutes. Standard modulus measurements were done on sheets in the hot rolled, cold rolled, and flash annealed conditions as listed in Table 21.
Tensile specimens were cut via EDM in the ASTM E8 subsize standard geometry. Tensile testing was performed on an Instron Model 3369 mechanical testing frame, using the Instron Bluehill control and analysis software. Samples were tested at room temperature under displacement control at a strain rate of 1×10−3 per second. Samples were mounted to a stationary bottom fixture, and a top fixture attached to a moving crosshead. A 50 kN load cell was attached to the top fixture to measure load. Tensile loading was performed to a load less than the yield point previously observed in tensile testing of the material, and this loading curve was used to obtain modulus values. Samples were pre-cycled under a tensile load below that of the predicted yield load to minimize the impact of grip settling on the measurements. Measurement results are shown in Table 22.
Measured values of the alloy modulus vary from 160 to 204 GPa depending on alloy chemistry and sample condition. Note that the as hot rolled modulus measurements were conducted on samples with a small degree of warp, which may lower the measured values.
This Case Example illustrates that Elastic Modulus of High Ductility Steel alloys depends on alloy chemistry and produced sheet condition and vary in the range from 160 GPa to 204 GPa.
Using commercial purity feedstock, a 3 kg charge of Alloy 44 was weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine that was then processed with a two-step hot rolling with corresponding parameters specified in Table 6. Hot rolled sheets were then subjected to further cold rolling in multiple passes, with a total reduction of approximately 25%. Rolling was done on a Fenn Model 061 single stage rolling mill. A list of specific cold rolling parameters used for the alloy is shown in Table 7. The tensile specimen tested in this study was annealed at 850° C. for 5 minutes, and then subsequently air cooled to room temperature. Tensile testing was conducted on an Instron 3369 Model test frame. Samples were mounted to a stationary bottom fixture, and a top fixture attached to a moving crosshead. A load cell was attached to the top fixture to measure load. The load limit of load cell was 50 kN. Strain was measured by using non-contact video extensometer. The resultant stress-strain curve is shown in
This Case Example illustrates extensive strain hardening in the High Ductility Steel alloys leading to high uniform ductility.
Using commercial purity feedstock, 3 kg charges of Alloy 141, Alloy 142 and Alloy 143 were weighed out according to the alloy stoichiometry in Table 4 and cast into a 50 mm thick laboratory slab in an Indutherm VTC800V vacuum tilt casting machine that was then processed with a two-step hot rolling at 1275° C. Hot rolled sheet from Alloy 141, Alloy 142 and Alloy 143 was further cold rolled to 1.18 mm thickness. Cold rolled sheet from both alloys was heat treated at 850° C. for 5 minutes.
To make SEM specimens, the cross-section samples of the sheets were cut and ground by SiC paper and then polished progressively with diamond media paste down to 1 μm grit. The final polishing was done with 0.02 μm grit SiO2 solution. The microstructure at the central layer of cross-section of sheet was observed, imaged, and evaluated. SEM microscopic analysis was conducted using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
This Case Example demonstrates structural development in the alloys in accordance with the path described in
The ability of High Ductility Steel alloys herein to undergo structural homogenization during deformation at elevated temperature, their structure and property reversibility during cold rolling/annealing cycles and capability in Mixed Microconstituent Structure formation (Structure #3,
Solidification of High Ductility Steel alloys without chemical segregation enable utilization of various casting methods that include but are not limited to mold casting, die casting, semi-solid metal casting, centrifugal casting. Modal Structure (Structure #1,
Thermo-mechanical treatment of cast products with Modal Structure (Structure #1,
Cold working of products with Homogenized NanoModal Structure (Structure #2,
This Case Example anticipates the potential processing routes for High Ductility Steel alloys herein towards final products for various applications based on their ability for structural homogenization during deformation at elevated temperature, structure and property reversibility during cold rolling/annealing cycles and capability to form Mixed Microconstituent Structure #3,
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/054,728, filed on September, 2014 and U.S. Provisional Patent Application Ser. No. 62/064,903, filed on Oct. 16, 2014, which are fully incorporated herein by reference.
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