Patent Application
20150090372
This application deals with a class of metal alloys with advanced property combinations applicable to metallic sheet production. More specifically, the present application identifies the formation of metal alloys of relatively high strength and ductility and the use of one or more cycles of elevated temperature treatment and cold deformation to produce metallic sheet at reduced thickness with relatively high strength and ductility.
Steels have 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 steel compositions and thermal processing, a wide variety of characteristic microstructures (i.e. polygonal ferrite, pearlite, bainite, austenite 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 nowadays.
Currently, there are over 25,000 worldwide equivalents in 51 different ferrous alloy metal groups. For steel produced in sheet form, broad classifications may be employed based on tensile strength characteristics. Low-Strength Steels (LSS) may be defined as exhibiting ultimate tensile strengths less than 270 MPa and include types such as interstitial free and mild steels. High-Strength Steels (HSS) may be steel defined as exhibiting ultimate tensile strengths from 270 to 700 MPa and include types such as high strength low alloy, high strength interstitial free and bake hardenable steels. Advanced High-Strength Steels (AHSS) steels may have ultimate tensile strengths greater than 700 MPa and include types such as martensitic steels (MS), dual phase (DP) steels, transformation induced plasticity (TRIP) steels, complex phase (CP) steels and twin induced plasticity (TWIP) 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 50%, respectively.
AHSS have been developed for automotive applications. See, e.g., U.S. Pat. Nos. 8,257,512 and 8,419,869. These steels are characterized by improved formability and crash-worthiness compared to conventional steel grades. Current AHSS are produced in processes involving thermo-mechanical processing followed by controlled cooling. To achieve the desired final microstructures in either uncoated or coated automotive products requires a control of a large number of variable parameters with respect to alloy composition and processing conditions.
Further developments of AHSS steels, designed for specific applications, will require careful control of alloying, microstructure and thermo-mechanical processing routes to optimize the specific strengthening and plasticity mechanisms responsible, respectively, for the desirable final strength and ductility characteristics.
The present disclosure is directed at alloys and their associated methods of production. The method comprises:
Optionally, one may then apply the following steps:
In the above, the solidified alloy in step (b) and step (c) may have a thickness in the range of 1 mm to 500 mm. In steps (d), (e) and (f), the thickness may be reduced to a desired level, without compromising the mechanical properties.
The present disclosure also relates to a method comprising:
In the above embodiment the heating and stressing of the alloy (step b) may be repeated in order to achieve a particular reduced thickness for the alloy that is targeted for a selected application. Accordingly, the alloys of the present disclosure have application to continuous casting processes including belt casting, thin strip/twin roll casting, thin slab casting and thick slab casting. The alloys find particular application in vehicles, drill collars, drill pipe, pipe casing, tool joint, wellhead, compressed gas storage tanks or liquefied natural gas canisters.
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 are such that they are initially capable of formation of what is described herein as Class 1 or Class 2 Steel which are preferably crystalline (non-glassy) with identifiable crystalline grain size morphology and mechanical properties. The present disclosure focuses upon improvements to the Class 2 Steel and the discussion below regarding Class 1 is intended to provide clarifying context.
The formation of Class 1 Steel herein is illustrated in
The Modal Structure of Class 1 Steel will therefore initially possess, 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 size of 25 nm to 5000 nm (i.e. non-metallic grains such as M2B where M is the metal and is covalently bonded to B). The borides 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. Note that the metal borides have been identified as exhibiting the M2B stoichiometry but other stoichiometry's are possible and may provide pinning including M3B, MB (M1B1), M23B6, and M7B3.
The Modal Structure of Class 1 Steel may be deformed by thermomechanical deformation and through heat treatment, resulting in some variation in properties, but the Modal Structure may be maintained.
When the Class 1 Steel noted above is exposed to a mechanical stress, the observed stress versus strain diagram is illustrated in
Reference to the hexagonal phases may be understood as a dihexagonal pyramidal class hexagonal phase with a P63mc space group (#186) and/or a ditrigonal dipyramidal class with a hexagonal P6bar2C space group (#190). In addition, 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 630 MPa to 1100 MPa, with an elongation of 10-40%. Furthermore, the second structure type of the Class 1 Steel is such that it exhibits a strain hardening coefficient between 0.1 to 0.4 that is nearly flat after undergoing the indicated yield. The strain hardening coefficient is reference to the value of n in the formula σ=K εn, where a represents the applied stress on the material, ε is the strain and K is the strength coefficient. 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 1 below provides a summary on structures and mechanisms in Class 1 Steel herein.
The formation of Class 2 Steel herein is illustrated in
As shown therein, Modal Structure (Structure #1) is initially formed as 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. Accordingly, Structure #1 of the Class 2 Steel may be preferably achieved by processing through either laboratory scale procedures as shown and/or through industrial scale methods involving chill surface processing methodology such as twin roll processing, thick or thin slab 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 200 nm to 200,000 nm containing austenite and/or ferrite; (2) boride sizes of 20 nm to 10000 nm (i.e. non-metallic grains such as M2B where M is the metal and is covalently bonded to B). The borides 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. Note that the metal borides have been identified as exhibiting the M2B stoichiometry but other stoichiometry's are possible and may provide pinning including M3B, MB (M1B1), M23B6, and M7B3 and which are unaffected by Mechanisms #1 or #2 noted above). Furthermore, Structure #1 of Class 2 steel herein includes austenite and/or ferrite along with such boride phases.
The Modal Structure is preferably first created (Structure #1,
Characteristic of the Nanophase Refinement (Mechanism #1,
Accordingly, grain coarsening does not occur with the alloys of Class 2 Steel herein during the Nanophase Refinement. Structure #2 is uniquely able to transform to Structure #3 during Dynamic Nanophase Strengthening (Mechanism #2,
Depending on alloy chemistries, nano-scale precipitates can form during 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 in size, 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 borides are found to be in a range from 20 to 10000 nm in size.
Expanding upon the above, in the case of the alloys herein that provide Class 2 Steel, when such alloys exceed their yield point, plastic deformation at constant stress occurs followed by a dynamic phase transformation leading toward the creation of Structure #3. More specifically, after enough strain is induced, an inflection point occurs where the slope of the stress versus strain curve changes and increases. In
With further straining during Dynamic Nanophase Strengthening, 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. Note that the strengthening transformation that occurs at the material straining under the stress generally defines Mechanism #2 as a dynamic process, leading to Structure #3. By “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 400 MPa to 1825 MPa and 1.0% to 59.2% 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.
With regards to this dynamic mechanism, new and/or additional precipitation phase or phases are observed that possesses identifiable grain sizes of 1 nm to 200 nm. In addition, there is the further identification in said precipitation phase of a dihexagonal pyramidal class hexagonal phase with a P63mc space group (#186), a ditrigonal dipyramidal class with a hexagonal P6bar2C space group (#190), and/or a M3Si cubic phase with a Fm3m space group (#225). Accordingly, the dynamic transformation can occur partially or completely and results in the formation of a microstructure with novel nanoscale/near nanoscale phases providing relatively high strength in the material. That is, Structure #3 may be understood as a microstructure having matrix grains sized generally from 25 nm to 2500 nm which are pinned by boride phases which are in the range of 20 nm to 10000 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 Nanophase Refinement and continues during Dynamic Nanophase Strengthening leading to Structure #3 formation. The volume fraction of the precipitation phase/grains of 1 nm to 200 nm in size in Structure #2 increases during transformation into Structure #3 and assists with the identified strengthening mechanism. It should also be noted that in Structure #3, the level of gamma-iron is optional and may be eliminated depending on the specific alloy chemistry and austenite stability.
Note that dynamic recrystallization is a known process but differs from Mechanism #2 (
As noted above, the steel alloys herein are such that they are capable of formation of High Strength Nanomodal Structure (Structure #3,
With reference to
In addition, as illustrated in
Expanding upon the above, when steel alloys with full or partial High Strength Nanomodal Structure (Structure #3) are subjected to high temperature exposure (temperatures greater than or equal to 700° C. but less than the melting point) recrystallization takes place leading to formation of Recrystallized Modal Structure (Structure #4,
The Recrystallized Modal Structure (Structure #4,
Expanding upon the above, in the case of straining of the alloys herein with the Recrystallized Modal Structure (Structure #4,
With regards to Mechanism #3) (
As shown by the arrows in
There are many examples regarding the use of the cyclic nature of these transformations in industrial processing. For example, consider a sheet with the chemistries and operable mechanisms and enabling microstructures which is cast initially at 50 mm thick by the thin slab process and then hot rolled through several steps to produce a 3 mm sheet. However, the sheet targeted gauge thickness is ˜1 mm for a particular application in an automobile. Thus, the as-hot rolled 3 mm thick sheet must then be cold rolled down to the targeted gauge. After 30% of reduction the 3 mm sheet is now ˜2.1 mm thick and has formed the High Strength Nanomodal Structure (Structure #3 in
The sheet is now heat treated (heating above 700° C. but below the Tm) and the Recrystallized Modal Structure (Structure #4) is formed. This sheet is then cold rolled another 30% of reduction to a gauge thickness of ˜1.5 mm and the formation of the Refined High Strength Nanomodal Structure (Structure #5). Further cold reduction would again result in breakage of the sheet. A heat treatment is then applied to recrystallize the sheet resulting in a high ductility Recrystallized Modal Structure (Structure #4). The sheet is then cold rolled another 30% to yield a gauge thickness of ˜1.0 mm thickness with a Refined High Strength Nanomodal Structure (Structure #5) obtained. After the gauge thickness target is reached, no further cold rolling reduction is necessary. Depending on the specific application, the sheet may or may not be heated again to be recrystallized. For example, for subsequent cold stamping of parts, it would be advantageous to recrystallize the sheet to form the high ductility Recrystallized Modal Structure (Structure #4). This resulting sheet may then be cold stamped by the end user and during the stamping process, would partially or completely transform into the Refined High Strength Nanomodal Structure (Structure #5).
Another example after forming the Recrystallized Modal Structure (Structure #4), in one or multiple steps, would be to expose this structure to cold deformation through cold rolling and after exceeding the yield strength to Nanophase Refinement and Strengthening (Mechanism #3). As a variant, however, the material could be only partially cold rolled and then not annealed (i.e. recrystallized). For example, a particular sheet material with the Recrystallized Modal Structure (Structure #4) which can be cold rolled up to 40% before breaking for example could instead be only cold rolled 10%, 20% or 30% and then not annealed. This would results in partial transformation through Nanophase Refinement and Strengthening (Mechanism #3) and would result in unique combinations of yield strength, ultimate tensile strength, and ductility which could be tailored for specific applications with different requirements. For example, high yield strength and high tensile strength is needed in a passenger compartment of an automobile to avoid impingement during a crash event while low yield strength and high tensile strength with high ductility might be quite attractive in use in the front or back end of the automobile in what is often termed the crash energy management zones.
It should now be appreciated that a specific feature herein is the ability of the steel alloys herein to undergo Nanophase Refinement & Strengthening (Mechanism #3) after forming the Recrystallized Modal Structure (Structure #4). An example of mechanical behavior of the steel alloys herein with Recrystallized Modal Structure (Structure #4) is schematically shown in
The chemical composition of the alloys studied is shown in Table 4 which provides the preferred atomic ratios utilized. Initial studies were done by sheet casting in a Pressure Vacuum Caster (PVC). Using high purity elements (>99 wt %), four 35 g alloy feedstock's of the targeted alloys were weighed out according to the atomic ratios provided in Table 4. 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. After mixing, the ingots were then placed in a PVC chamber, melted using RF induction and then ejected onto a copper die designed for casting 3 inch by 4 inch sheets with thickness of 3.3 mm.
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 four elements at the following indicated atomic percent: Fe (55.0 to 88.0 at. %); B (0.50 to 8.0 at. %); Si (0.5 to 12.0 at. %); Mn (1.0 to 19.0 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 to 9.0 at. %); Cr (0.1 to 19.0 at. %); Cu (0.1 to 6.00 at. %); Ti (0.1 to 1.00 at. %); C (0.1 to 4.0 at. %). Impurities may be present including atoms such as Al, Mo, Nb, S, O, N, P, W, Co, Sn, Zr, Pd and V, which may be present up to 10 atomic percent.
Accordingly, the alloys may herein also be more broadly described as Fe-based alloys (with Fe content greater than 50.0 atomic percent) and further including B, Si and Mn, and capable of forming Class 2 steel (
Thermal analysis was performed on material in the as cast state for all alloys of interest. Measurements were taken 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 1400° C. at a rate of 10° C./minute, a controlled cooling from 1400° C. to 900° C. at a rate of 10° C./min, and a second heating to 1400° 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 ˜1120° C. depending on alloy chemistry and final melting temperature exceeding 1425° C. in some instances (marked N/A in Table 5). Accordingly, the melting point range for the alloys herein capable of Class 2 Steel formation and subsequent recrystallization and cold forming (
The density of the alloys was measured on arc-melt ingots 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 6 and was found to vary from 7.30 g/cm3 to 7.89 g/cm3. Experimental results have revealed that the accuracy of this technique is ±0.01 g/cm3.
Plates from each alloy from Alloy 1 to Alloy 283 was subjected to Hot Isostatic Pressing (HIP) using an American Isostatic Press Model 645 machine with a molybdenum furnace and with a furnace chamber size of 4 inch diameter by 5 inch height. The plates were heated at 10° C./min until the target temperature was reached and were exposed to gas pressure for specified time which was held at 1 hour for these studies. HIP cycle parameters are listed in Table 7. The key aspect of the HIP cycle was to remove macrodefects such as pores and small inclusions by mimicking hot rolling during sheet production by Thin Strip/Twin Roll Casting process or Thick/Thin Slab Casting process. The HIP cycle, which is a thermomechanical process allows the elimination of some fraction of internal and external macrodefects while smoothing the surface of the plate.
After HIP cycle, the plates were heat treated at parameters specified in Table 8. 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, modeling coiling conditions at commercial sheet production. In cases of controlled cooling, the furnace temperature was lowered at a specified rate, with samples loaded, allowing for a control of the sample cooling rate.
The tensile specimens were cut from the plates after HIP cycle 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. 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 after HIPing are listed in Table 9 and this relates to Structure 3 noted above. The ultimate tensile strength values vary from 403 to 1810 MPa with tensile elongation from 1.0 to 33.6%. The yield strength is in a range from 205 to 1223 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry and processing/treatment condition.
Cast plates from selected alloys listed in Table 4 were thermo-mechanically processed via hot rolling. The plates were heated in a tunnel furnace to a target temperature equal to the nearest 25° C. temperature interval that was at least 50° C. below the solidus temperature previously determined (see Table 5). The rolls for the mill were held at a constant spacing for all samples rolled, such that the rolls were touching with minimal force. The resulting reductions varied between 21.0% and 41.9%. The primary importance of the hot rolling stage is to initiate Nanophase Refinement and to remove macrodefects such as pores and voids by mimicking the hot rolling at Stage 2 of Twin Roll Casting process or at Stage 1 or Stage 2 of Thin Slab Casting process. This process eliminates a fraction of internal macrodefects, in addition to smoothing out the sample surface. After hot rolling, the plates were heat treated at parameters specified in Table 8. The tensile specimens were cut from the plates 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. 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. Samples were tested in the as-rolled state and after heat treatments defined in Table 8.
Tensile properties of selected alloys herein with Nanomodal Structure (Structure #2,
Heat treatment after hot rolling leads to further development of Nanomodal Structure (Structure #2) that transforms into High Strength Nanomodal Structure (Structure #3) during deformation. Tensile properties of the selected alloys after hot rolling and heat treatment at different parameters are listed in Table 10. The ultimate tensile strength values may vary from 730 to 1435 MPa with tensile elongation from about 2 to 59.2%. The yield strength is in a range from 274 to 1020 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry and processing/treatment condition.
Selected alloys from Table 4 were cast into plates with thickness of 50 mm using 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 (
Cast plates with initial thickness of 50 mm were subjected to hot rolling at the temperatures between 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 correct for temperature lost during the 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%.
Tensile specimens were cut from hot rolled sheets via EDM. 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 11. The ultimate tensile strength values may vary from 978 to 1281 MPa with tensile elongation from 14.0 to 29.2%. The yield stress is in a range from 396 to 746 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry and hot rolling conditions.
Hot-rolled sheets from each alloy were then subjected to further cold rolling in multiple passes down to thickness of 1.2 mm. Rolling was done on a Fenn Model 061 single stage rolling mill. Tensile properties of the alloys after hot rolling and subsequent cold rolling are listed in Table 12. The ultimate tensile strength values in this specific example may vary from 1438 to 1787 MPa with tensile elongation from 1.0 to 20.8%. The yield stress is in a range from 809 to 1642 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry and processing conditions. Cold rolling reduction influences the amount of austenite transformation leading to different level of strength in the alloys.
After cold rolling, alloys were heat treated at the parameters specified in Table 13. 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 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 selected alloys after hot rolling with subsequent cold rolling and heat treatment at different parameters are listed in Table 14. The ultimate tensile strength values in this specific case example may vary from 813 MPa to 1316 MPa with tensile elongation from 6.6 to 35.9%. The yield stress is in a range from 274 to 815 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry and processing conditions.
Industrial sheet from selected alloys was produced by Thin Strip Casting process. A schematic of the Thin Strip Casting process is shown in
This Case Example demonstrates that the alloys provided for in Table 4 are applicable for industrial processing through continuous casting processes.
In order to get targeted sheet thickness and optimized properties for different applications, produced sheet undergoes post-processing. To simulate post-processing conditions at industrial production, sheet strips with approximate size of 4 inches by 6 inches were cut from the industrial sheet produced by Thin Strip Casting process and then post-processed by various approaches. A summary of the various approaches used from several hundreds of experiments with variations noted is provided below.
To simulate the hot rolling process, the strips were subjected to rolling using a Fenn Model 061 Rolling Mill and a Lucifer 7-R24 Atmosphere Controlled Box Furnace. The plates were placed in a hot furnace typically from 850 to 1150° C. for 10 to 60 minutes prior to the start of rolling. The strips were then repeatedly rolled at between 10% and 25% reduction per pass and were placed in the furnace for 1 to 2 min between rolling steps to allow then to return to temperature. If the plates became too long to fit in the furnace they were cooled, cut to a shorter length, then reheated in the furnace for additional time before they were rolled again.
To simulate the cold rolling process, the strips were subjected to cold rolling using a Fenn Model 061 Rolling Mill with different reduction depending on the post-processing goal. To reduce sheet thickness, reduction of 10 to 15% per pass with typically 25 to 50% total was applied before intermediate annealing at various temperatures (800 to 1170° C.) and various times (2 minutes to 16 hours). To mimic the skin pass step for final production, sheet was cold rolled with reduction typically from 2 to 15%. Heat treatment studies were done by using a Lindberg Blue M Model “BF51731C-1” Box Furnace in air to simulate in-line annealing on a hot dip pickling line with temperatures typically from 800 to 1200° C. and times from typically 2 minutes to 15 minutes. To mimic coil batch annealing conditions, a Lucifer 7-R24 Atmosphere Controlled Box Furnace was utilized for heat treatments with temperatures typically from 800 to 1200° C. and times from typically 2 hours up to 1 week.
This case Example demonstrates that the alloys in Table 4 are applicable to the various post processing steps used industrially.
Industrial sheet from Alloy 260 and Alloy 284 was produced by Thin Strip Casting process. As-solidified thickness of the sheet was 3.2 and 3.6 mm, respectively (corresponds to Stage 1 of Thin Strip Casting process,
Samples from Alloy 260 industrial sheet were post-processed to mimic processing at commercial scale including (1) homogenization heat treatment at 1150° C. for 2 hr; (2) cold rolling with reduction of 15%; (3) annealing at 1150° C. for 5 min and skin pass with 5% reduction. The tensile specimens were cut from the sheets using a Brother HS-3100 wire electrical discharge machining (EDM). The 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 with the load cell attached to the top fixture.
Properties of the Alloy 260 sheet at each step of post-processing are shown in
Samples from Alloy 284 industrial sheet were also post-processed to mimic processing at commercial scale with different post-processing parameters. The post-processing includes (1) homogenization heat treatment at 1150° C. for 2 hr; (2) homogenization heat treatment at 1150° C. for 2 hr+cold rolling with 45% reduction+annealing at 1150° C. for 5 min; (3) homogenization heat treatment at 1150° C. for 8 hr+cold rolling with 15% reduction+annealing at 1150° C. for 5 min; (4) homogenization heat treatment at 1150° C. for 8 hr+cold rolling with 25% reduction+annealing at 1150° C. for 2 hr; (5) homogenization heat treatment at 1150° C. for 16 hr+cold rolling with 25% reduction+annealing at 1150° C. for 5 min. Structural development in the Alloy 284 sheet is similar to that in Alloy 260 sheet as described above for each step of post-processing and the intermediate step properties are not provided here. The resultant Alloy 284 sheet properties after these post-processing routes are shown in
This case Example demonstrates the enabling of advanced property combinations in sheet alloys herein in the fully post processed condition. Structure development in both alloys herein follows the pattern outlined in
Modal Structure specified as Structure #1 (
Structural analysis was performed by scanning electron microscopy (SEM) using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc. To make SEM specimens, the cross-section of the as-cast sheet was cut and ground by SiC paper and then polished progressively with diamond media suspension down to 1 μm grit. The final polishing was done with 0.02 μm grit SiO2 solution. SEM images of microstructure in the outer layer region that is close to the surface and in the central layer region of the as-solidified sheet samples are shown in
As demonstrated in this Case Example, Modal Structure (Structure #1) forms in steel alloys herein at solidification during laboratory and industrial casting processes.
When Modal Structure (Structure #1) is subjected to high temperature exposure, it transforms into Nanomodal Structure (Structure #2) through Nanophase Refinement (Mechanism #1). To illustrate this, samples were cut from the Alloy 260 industrial sheet produced by Thin Strip Casting process with in-line hot rolling (32% reduction) that were heat treated at 1150° C. for 2 hours, and then cooled to room temperature in air. Samples for various studies including tensile testing, SEM microscopy, TEM microscopy, and X-ray diffraction were cut after heat treatment using a wire-EDM.
SEM samples were cut out from the heat treated sheet from Alloy 260 and metallographically polished in stages down to 0.02 μm Grit to ensure smooth samples for scanning electron microscopy (SEM) analysis. SEM was done using a Zeiss EVO-MA10 model with the maximum operating voltage of 30 kV. Example SEM backscattered electron micrographs of the microstructure in the Alloy 260 sheet samples after heat treatment are shown in
To examine the structural details of the Alloy 260 industrial sheet in more detail, high resolution transmission electron microscopy (TEM) was utilized. To prepare TEM specimens, samples were cut from the heat-treated industrial sheets. The samples were then ground and polished to a thickness of 70 to 80 μm. Discs of 3 mm in diameter were punched from these thin samples, and the final thinning was done by twin-jet electropolishing using a mixture of 30% HNO3 in methanol base. The prepared specimens were examined in a JEOL JEM-2100 HR Analytical Transmission Electron Microscope (TEM) operated at 200 kV. TEM micrographs of the microstructure in the Alloy 260 industrial sheet samples after heat treatment at 1150° C. for 2 hr are shown in
As demonstrated in this Case Example, Nanomodal Structure (Structure #2,
Industrial sheet from Alloy 260 produced by Thin Strip Casting and heat treated at 1150° C. for 2 hours was cold rolled using a Fenn Model 061 Rolling Mill mimicking the cold rolling step at industrial post processing of the produced steel sheet. The microstructure of the cold rolled samples was studied by SEM. To make SEM specimens, the cross-sections of the hot rolled 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 cold rolled samples from Alloy 260 sheets were examined by scanning electron microscopy (SEM) using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
The TEM images of the microstructure in the cold rolled sample are shown in
Additional details of the Alloy 260 sheet structure including the nature of the small nanocrystalline phases were revealed by 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 40 kV with a filament current of 40 mA. The scans was 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 scan was then subsequently analyzed by Rietveld analysis using Siroquant software. In
As demonstrated in this Case Example, the High Strength Nanomodal Structure (Structure #3,
Following 50% cold rolling, industrial sheet from Alloy 260 was heat treated at 1150° C. for 2 and 5 minutes to mimic in-line induction annealing of steel sheet as well as for 2 hours to mimic the batch annealing of industrial coils. Samples were cut from heat treated sheet and metallographically polished in stages down to 0.02 μm grit to ensure smooth samples for scanning electron microscopy (SEM) analysis. SEM was done using a Zeiss EVO-MA10 model with the maximum operating voltage of 30 kV. Example SEM backscattered electron micrographs of the microstructure in the sheet from Alloy 260 after cold rolling and heat treatment at two conditions are shown in
As shown in
Samples from Alloy 260 sheet that were heat treated at 1150° C. for 5 minutes and 2 hr were studied by TEM. TEM specimen preparation procedure includes cutting, thinning, and electropolishing. First, samples were cut with electric discharge machine, and then thinned by grinding with pads of reduced grit size every time. Further thinning to 60 to 70 μm thickness is 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 mixture of 30% nitric acid 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 usually was done at 4.5 keV, and the inclination angle is 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.
After heat treatment at 1150° C., the cold rolled samples show extensive recrystallization. As shown in
Additional details of the Recrystallized Modal Structure in the Alloy 260 sheet were revealed by 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 40 kV with a filament current of 40 mA. The scan was 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 scan was then subsequently analyzed using Rietveld analysis using Siroquant software. In
As demonstrated in this Case Example, Recrystallized Modal Structure (Structure #4,
Microstructure of industrial sheet from Alloy 260 with Recrystallized Modal Structure (Structure #4,
TEM specimen preparation procedure includes cutting, thinning, and electropolishing. First, samples were cut using electric discharge machining from the gage section of tensile specimens, and then thinned by grinding with pads of reduced grit size media every time. Further thinning to 60 to 70 μm thick is 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.
Additional details of the microstructure in the gage section of tensile specimens from Alloy 260 sheet were revealed by 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 40 kV with a filament current of 40 mA. The scan was 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 scan was then subsequently analyzed using Rietveld analysis using Siroquant software. In
As demonstrated in this Case Example, Recrystallized Modal Structure (Structure #4,
Industrial sheet from Alloy 260 was produced by the Thin Strip Casting process. As-solidified thickness of the sheet was 3.2 mm (corresponds to Stage 1 of the Thin Strip Casting process,
Tensile properties were measured of sheet material in the as hot rolled, overaged, cold rolled, and annealed states. The tensile properties were tested 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 with the load cell attached to the top fixture. Video extensometer was utilized for strain measurements. Tensile properties for industrial sheet from Alloy 260 after overaging heat treatment at 1150° C. for 8 hours and 16 hours and following steps of post-processing are shown in
This Case Example demonstrates that overaging of the sheet leads to grain coarsening that results in property reduction. However, this damaged microstructure transforms into Refined High Strength Nanomodal Structure (Structure #5,
Industrial sheet from Alloy 284 was produced by Thin Strip Casting process with an as-solidified thickness of 3.2 mm (corresponds to Stage 1 of the Thin Strip Casting process,
Tensile properties were measured of Alloy 284 sheet in the as hot rolled, overaged, cold rolled, and annealed states. The tensile properties were tested 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 with the load cell attached to the top fixture. Video extensometer was utilized for strain measurements. Tensile properties for industrial sheet from Alloy 284 after overaging heat treatment at 1150° C. for 8 hours are shown in
This Case Example demonstrates that overaging of the sheet leads to grain coarsening that results in property reduction. However, this damaged microstructure transforms into Refined High Strength Nanomodal Structure (Structure #5,
Industrial sheet from Alloy 260 was produced by the Thin Strip Casting process. As-solidified thickness of the sheet was 3.45 mm (corresponds to Stage 1 of the Thin Strip Casting process,
Tensile properties were measured of the Alloy 260 sheet in the as hot rolled, heat treated, cold rolled, and annealed states. The tensile properties were tested 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 with the load cell attached to the top fixture. Video extensometer was utilized for strain measurements. Tensile properties for Alloy 260 in the initial (as hot rolled and after step 1) and final (after step 6 and 7) state are shown in
As shown by this Case Example, this process of strain hardening during cold working, followed by recrystallization during annealing, followed by strain hardening by cold rolling again can be applied multiple times as necessary to hit the final gauge thickness target and provide targeted properties in the sheet.
In order to produce sheet with different thicknesses, cold rolling gauge reduction followed by annealing is used by the steel industry. This process includes the use of cold rolling mills to mechanically reduce the gauge thickness of sheet with intermediate in-line or batch annealing between passes to remove the cold work present in the sheet.
The cold rolling gauge reduction and annealing process was simulated for Alloy 260 material that was commercially produced by the Thin Strip casting process. Alloy 260 was cast at 3.65 mm thickness, and reduced 25% via hot rolling at 1150° C. to 2.8 mm thickness. Following hot rolling, the sheet was coiled and annealed in an industrial batch furnace for a minimum of 2 hours at 1150° C. at the coolest part of the coil. The gauge thickness of the sheet was reduced by 13% in one cold rolling pass by tandem mill, then annealed in-line at 1100° C. for 2 to 5 min. The sheet gauge thickness was further reduced by 25% in 4 cold rolling passes by reversing mill to approximately 1.8 mm in thickness and annealed in an industrial batch furnace at 1100° C. for 30 minutes at the coolest part of the coil (i.e. inner windings). Resultant commercially produced sheet with 1.8 mm thickness was used for further cold rolling in multiple steps using a Fenn Model 061 Rolling Mill with intermediate annealing as described in Table 21. All anneals were completed using a Lucifer 7-R24 box furnace with flowing argon. During anneals, the sheet was loosely wrapped in stainless steel foil to reduce the potential of oxidation from atmospheric oxygen.
Tensile properties of the Alloy 260 sheet were measured at each step of processing. Tensile samples were cut using a Brother HS-3100 wire EDM. The tensile properties were tested 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 ridged and the top fixture moving with the load cell attached to the top fixture. Video extensometer was utilized for strain measurements. Tensile properties of commercially produced 1.8 mm thick sheet and after each step of processing specified in Table 17 are shown below in Table 18 and illustrated in
The tensile properties shown in
This Case Example demonstrates that the cold rolling gage reduction and annealing process can be used cyclically while transitioning between the Refined High Strength Nanomodal Structure (Structure #5,
The ability of the steel alloys herein to form Recrystallized Modal Structure (Structure #4) that undergoes Nanophase Refinement and Strengthening (Mechanism #3) during deformation leading to advanced property combination enables sheet production by different methods including belt casting, thin strip/twin roll casting, thin slab casting, and thick slab casting with achievement of advanced property combination by subsequent post-processing with realization of new enabling mechanisms herein. While thin strip casting was mentioned previously, a short description of the slab casting processes is provided below. Note that the front end of the process of forming the liquid melt of the alloy in Table 4 is similar in each process. One route is starting with scrap which can then be melted in an electric arc furnace (EAF), followed by argon oxygen decarburization (AOD) furnace, and the final alloying through a ladle metallurgy furnace (LMF) treatment. Additionally, the back end of the process for each production process is similar as well, in-spite of the large variation in as-cast thickness. Typically, the last step of hot rolling results, in the production of hot rolled coils with thickness from 1.5 to 10 mm which is dependent on the specific process flow and goals of each steel producer. For the specific chemistries of the alloys in this application and the specific structural formation and enabling mechanisms as outlined herein, the resulting structure of these as-hot rolled coils would be the Structure #2 (Nanomodal Structure). If thinner gauges are then needed, cold rolling of the hot rolled coils is typically done to produce final gauge thickness which may be in the range of 0.2 to 3.5 mm in thickness). It is during these cold rolling gauge reduction steps, that the new structures and mechanisms as outlined in
As explained previously and shown in the case examples, the process of High Strength Nanomodal Structure formation, recrystallization into the Recrystallized Modal Structure, and refinement and strengthening through NanoPhase Refinement & Strengthening into the Refined
High Strength Nanomodal Structure can be applied in a cyclic nature as often as necessary in order to reach end user gauge thickness requirements typically 0.1 to 25 mm thickness for Structures #3, #4 or #5.
Thick slab casting is the process whereby molten metal is solidified into a “semifinished” slab for subsequent rolling in the finishing mills. In the continuous casting process pictured in
In the case of thin slab casting, the steel is cast directly to slabs with a thickness between 20 and 150 mm. The method involves pouring molten steel into the Tundish at the top of the slab caster, from a ladle. They are sized with a working volume of about 100 t, which will deliver the steel at a rate of one ladle every 40 minutes to the caster. The temperatures of liquid steel in the tundish as well as the steel purity and chemical composition have a significant impact on the quality of the cast product. The liquid steel passes at a controlled rate into the caster, which is made up of a water cooled mould in which the outer surface of the steel solidifies. In general, the slabs leaving the caster are about 70 mm thick, 1000 mm wide and approximately 40 m long. These are then cut by the shearer to length. To enable ease of casting a hydraulic oscillator and electromagnetic brakes are fitted to control the molten liquid whilst in the mould.
A schematic of the Thin Slab Casting process is shown in
| Number | Date | Country | |
|---|---|---|---|
| 61885842 | Oct 2013 | US |
