Patent Application
20150114587
This application deals with metal alloys and methods of processing with application to slab casting methods with post processing steps towards sheet production. These metals provide unique structures and exhibit advanced property combinations of high strength and/or high 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 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 nowadays.
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 understood herein as exhibiting tensile strengths less than 270 MPa and include types such as interstitial free and mild steels. High-Strength Steels (HSS) may be understood herein as exhibiting 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 be understood herein as having tensile strengths greater than 700 MPa and include types such 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 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 the process whereby molten metal is solidified into a “semifinished” billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary molds to form ingots. Since then, “continuous casting” has evolved to achieve improved yield, quality, productivity and cost efficiency. It allows lower-cost production of metal sections with better quality, due to the inherently lower costs of continuous, standardized production of a product, as well as providing increased control over the process through automation. This process is used most frequently to cast steel (in terms of tonnage cast). Continuous casting of slabs with either in-line hot rolling mill or subsequent separate hot rolling is important post processing steps to produce coils of sheet. Thick slabs are typically cast from 150 to 500 mm thick and then allowed to cool to room temperature. Subsequent hot rolling of the slabs after preheating in tunnel furnaces is done is several stages through both roughing and hot rolling mills to get down to thicknesses typically from 2 to 10 mm in thickness. Thin slab castings starts with an as-cast thickness of 20 to 150 mm and then is usually followed through in-line hot rolling in a number of steps in sequence to get down to thicknesses typically from 2 to 10 mm. There are many variations of this technique such as casting at thicknesses of 100 to 300 mm to produce intermediate thickness slabs which are subsequently hot rolled. Additionally, other casting processes are known including single and double belt casting processes which produce as-cast thickness in the range of 5 to 100 mm in thickness and which are usually in-line hot rolled to reduce the gauge thickness to targeted levels for coil production. In the automotive industry, forming of parts from sheet materials from coils is accomplished through many processes including bending, hot and cold press forming, drawing, or further shape rolling.
The present disclosure is directed at alloys and their associated methods of production. The method comprises:
Optionally, the alloy in step (c) may undergo one of the following additional steps: (1) stressing above the alloy's yield strength of 200 MPa to 1000 MPa and providing a resulting alloy that indicates a yield strength of 200 MPa to 1650 MPa, tensile strength of 400 MPa to 1825 MPa, and an elongation of 2.4% to 78.1%; or (2) heat treating the alloy to a temperature of 700° C. to 1200° C. to form an alloy having one of the following: matrix grains of 50 nm to 50000 nm; boride grains of 20 nm to 10000 nm (optional—not required); or precipitation grains with size of 1 nm to 200 nm. Such alloy with such morphology after heat treatment may then be stressed above its yield strength to form an alloy having yield strength of 200 MPa to 1650 MPa, tensile strength of 400 MPa to 1825 MPa and an elongation of 2.4% to 78.1%.
Accordingly, the alloys of 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, such as vehicle frames, 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.
A slab is a length of metal that is rectangular in cross-section. Slabs can be produced directly by continuous casting and are usually further processed via different processes (hot/cold rolling, skin rolling, batch heat treatment, continuous heat treatment, etc.). Common final products include sheet metal, plates, strip metal, pipes, and tubes.
Thick Slab Casting Description
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
Thin Slab Casting Description
A schematic of the thin slab casting process is shown in
While the three stage process of forming sheet in thin slab casting is part of the process, the response of the alloys herein to these stages is unique based on the mechanisms and structure types described herein and the resulting novel combinations of properties.
Hot Rolling
Hot rolled steel is formed to shape while it is red-hot then allowed to cool. Flat rolling is the most basic form of rolling with the starting and ending material having a rectangular cross-section. The schematic illustration of a rolling process for metal sheets is presented in
Cold Rolling
Cold rolled steel is made by passing cold steel material through heavy rollers which compress the metal to its final shape and dimension. It is a common step of post-processing during sheet production when different cold rolling mills can be utilized depending on material properties, cold rolling objective and targeted parameters. When sheet material undergoes cold rolling, its strength, hardness as well as the elastic limit increase. However, the ductility of the metal sheet decreases due to strain hardening thus making the metal more brittle. As such, the metal must be annealed/heated from time to time between passes during the rolling operation to remove the undesirable effects of cold deformation and to increase the formability of the metal. Thus obtaining large thickness reduction can be time and cost consuming. In many cases, multi-stand cold rolling mills with in-line annealing are utilized wherein the sheet is affected by elevated temperature for a short period of time (usually 2 to 5 min) by induction heating while it moves along the rolling line. Cold rolling allows a much more precise dimensional accuracy and final sheet products have a smoother surface (better surface finish) than those from hot rolling.
Heat Treatment
To get the targeted mechanical properties, post-processing annealing of the sheet materials is usually implemented. Typically, annealing of steel sheet products is performed in two ways at a commercial scale: batch annealing or continuous annealing. During a batch annealing process, massive coils of the sheet slowly heat and cool in furnaces with a controlled atmosphere. The annealing time can be from several hours to several days. Due to the large mass of the coils which may be typically 5 to 25 ton in size, the inside and outside parts of the coils will experience different thermal histories in a batch annealing furnace which can lead to differences in resulting properties. In the case of a continuous annealing process, uncoiled steel sheets pass through heating and cooling equipment for several minutes. The heating equipment is usually a two-stage furnace. The first stage is high temperature heat treatment which provides recrystallization of microstructure. The second stage is low temperature heat treatment and it offers artificial ageing of microstructure. A proper combination of the two stages of overall heat treatment during continuous annealing provides the target mechanical properties. The advantages of continuous annealing over conventional batch annealing are the following: improved product uniformity; surface cleanliness and shape; ability to produce a wide range of steel grades.
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 and morphology. The present disclosure focuses upon improvements to the Class 2 Steel and the discussion below regarding Class 1 is intended to provide initial context.
Class 1 Steel
The formation of Class 1 Steel herein is illustrated in
The modal structure of Class 1 Steel will therefore initially indicate, when cooled from the melt, the following grain sizes: (1) matrix grain size of 500 nm to 20,000 nm containing austenite and/or ferrite; (2) boride grain size of 25 nm to 5000 nm (i.e. non-metallic grains such as M2B where M is the metal and is covalently bonded to B). The boride grains may also preferably be “pinning” type phases which is reference to the feature that the matrix grains will effectively be stabilized by the pinning phases which resist coarsening at elevated temperature. Note that the metal boride grains have been identified as exhibiting the M2B stoichiometry but other stoichiometry is possible and may provide pinning including M3B, MB (M1B1), M23B6, and M7B3.
The Modal Structure of Class 1 Steel may be deformed by thermo-mechanical processes and undergo various heat treatments, resulting in some variation in properties, but the Modal Structure may be maintained.
When the Class 1 Steel noted above is exposed to a tensile 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 1150 MPa, with an elongation of 10 to 40%. Furthermore, the second type structure 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 σ 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 comparison and performance summary for Class 1 Steel herein.
Class 2 Steel
The formation of Class 2 Steel herein is illustrated in
As shown therein, Structure #1 is initially formed in which Modal Structure is the result of starting with a liquid melt of the alloy and solidifying by cooling, which provides nucleation and growth of particular phases having particular grain sizes. Grain size herein may again be understood as the size of a single crystal of a specific particular phase preferably identifiable by methods such as scanning electron microscopy or transmission electron microscopy. 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 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 grain sizes, if present, of 10 nm to 5000 nm (i.e. non-metallic grains such as M2B where M is the metal and is covalently bonded to B). The boride grains may also preferably be “pinning” type phases which are referenced to the feature that the matrix grains will effectively be stabilized by the pinning phases which resist coarsening at elevated temperature. Note that the metal boride grains have been identified as exhibiting the M2B stoichiometry but other stoichiometry is possible and may provide pinning including M3B, MB (M1B1), M23B6, and M7B3 and which are unaffected by Mechanisms #1 or #2 noted above. Reference to grain size is again to be understood as the size of a single crystal of a specific particular phase preferably identifiable by methods such as scanning electron microscopy or transmission electron microscopy. Furthermore, Structure #1 of Class 2 steel herein includes austenite and/or ferrite along with such boride phases.
In
Characteristic of the Static Nanophase Refinement (Mechanism #1) in Class 2 steel, if borides are present, is such that the micron scale austenite phase (gamma-Fe) which was noted as falling in the range of 200 nm to 200,000 nm is partially or completely transformed into new phases (e.g. ferrite or alpha-Fe) at elevated temperature. The volume fraction of ferrite (alpha-iron) initially present in the modal structure (Structure 1) of Class 2 steel is 0 to 45%. The volume fraction of ferrite (alpha-iron) in Structure #2 as a result of Static Nanophase Refinement (Mechanism #2) is typically from 20 to 80% at elevated temperature and then reverts back to austenite (gamma-iron) upon cooling to produce typically from 20 to 80% austenite at room temperature. The static transformation preferably occurs during elevated temperature heat treatment and thus involves a unique refinement mechanism since grain coarsening rather than grain refinement is the conventional material response at elevated temperature.
Accordingly, if borides are present, grain coarsening does not occur with the alloys of Class 2 Steel herein during the Static Nanophase Refinement mechanism. Structure #2 is uniquely able to transform to Structure #3 during Dynamic Nanophase Strengthening and as a result Structure #3 is formed and indicates tensile strength values in the range from 400 to 1825 MPa with 2.4 to 78.1% total elongation.
Depending on alloy chemistries, nanoscale precipitates can form during Static Nanophase Refinement and the subsequent thermal process in some of the non-stainless high-strength steels. The nano-precipitates are in the range of 1 nm to 200 nm, with the majority (>50%) of these phases 10˜20 nm in size, which are much smaller than matrix grains or the boride pinning phase formed in Structure #1 for retarding matrix grain coarsening when present. Also, during Static Nanophase Refinement, the boride grains, if present, 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 (
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 in 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 to 1825 MPa and 2.4% to 78.1% 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.
Thus, depending on the level of transformation, tunable yield strength may also now be developed in Class 2 Steel herein depending on the level of deformation and in Structure #3 the yield strength can ultimately vary from 200 MPa to 1650 MPa. That is, conventional steels outside the scope of the alloys here exhibit only relatively low levels of strain hardening, thus their yield strengths can be varied only over small ranges (e.g., 100 to 200 MPa) depending on the prior deformation history. In Class 2 steels herein, the yield strength can be varied over a wide range (e.g. 200 to 1650 MPa) as applied to the Structure #2 transformation into Structure #3, allowing tunable variations to enable both the designer and end users in a variety of applications, and utilize Structure #3 in various applications such as crash management in automobile body structures.
With regards to this dynamic mechanism shown in
Pathways for the development of High Strength Nanomodal Structure formation are as noted described in
The Dynamic Nanophase Refinement leading to the Homogenized Modal Structure is observed to occur in as little as 1 cycle (heating with thickness reduction) or after multiple reduction cycles of thickness (e.g. up to 25). The Homogenized Modal Structure (Structure 1a in
It is worth noting that Dynamic Nanophase Refinement (Mechanism #0) is a mechanism providing Homogenized Modal Structure (Structure #1a) in cast alloys preferably through the entire volume/thickness that makes the alloys effectively cooling rate insensitive (as well as thickness insensitive) during the initial solidification from the liquid state that enables utilization of such production methods as thin slab or thick slab casting for sheet production. In other words, it has been observed that if one forms Modal Structure at a thickness of greater than or equal to 2.0 mm or applies a cooling rate during formation of Modal Structure that is less than or equal to 250 K/s, the ensuing step of Static Nanophase Refinement may not readily occur. Therefore the ability to produce Nanomodal Structure (Structure #2) and accordingly, the ability to undergo Dynamic Nanophase Strengthening (Mechanism #2) and form High Strength Nanomodal Structure (Structure #3) will be compromised. That is the refinement of the structure will either not occur leading to properties which are either equivalent to those obtained from the Modal Structure or will be ineffective leading to properties which are between that of the Modal and NanoModal Structures.
However, one may now preferably ensure the ability to form Nanomodal Structure (Structure #2) and the ensuing development of High Strength Nanomodal Structure. More specifically, when starting with Modal Structure that is solidified from the melt with a thickness of greater than or equal to 2.0 mm or Modal Structure cooled at a rate of less than or equal to 250 K/s), one may now preferably proceed with Dynamic Nanophase Refinement (Mechanism #0) into Homogenized Modal Structure and then proceed with the steps illustrated in
As therefore identified, Dynamic Nanophase Refinement occurs after the alloys are subjected to deformation at elevated temperature and preferably occurs at a range from 700° C. to a temperature just below the melting point and over a range of strain rates from 10−6 to 104 s−1. One example of such deformation may occur by hot rolling after thick slab or thin slab casting which may occur in single or multiple roughing hot rolling steps or single and/or single or multiple finishing hot rolling steps. Alternatively it can occur at post processing with a wide variety of hot processing steps including but not limited to hot stamping, forging, hot pressing, hot extrusion, etc.
The formation of Modal Structure (Structure #1) in steel alloys herein can occur during alloy solidification at Thick Slab (
Integrated hot rolling of Thick Slab (
Mechanism #1 which is the Static Nanophase Refinement with Nanomodal Structure formation (Structure #2) occurs when produced slabs with Homogenized Modal Structure (Structure #1a,
Cold rolling is a widely used method for sheet production that is utilized to achieve targeted thickness for particular applications. For example, most sheet steel used for automotive industry has thickness in a range from 0.4 to 4 mm. To achieve targeted thickness, cold rolling is applied through multiple passes with intermediate annealing between passes. Typical reduction per pass is 5 to 70% depending on the material properties. The number of passes before the intermediate annealing also depends on materials properties and its level of strain hardening at cold deformation. Cold rolling is also used as a final step for surface quality known as a skin pass. For the steel alloys herein and through methods to form Nanomodal Structure as provided 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 plate casting in copper die.
Alloy 1 through Alloy 59 were cast into plates with thickness of 3.3 mm. Using commercial purity feedstock, 35 g alloy feedstocks 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. Individually, the ingots were disc-shaped, with a diameter of approximately 30 mm and a thickness of approximately 9.5 mm at the thickest point. The resulting ingots were then placed in a pressure vacuum caster (PVC) chamber, melted using RF induction and then ejected onto a copper die designed for casting 3 by 4 inches sheets with thickness of 3.3 mm.
Alloy 60 through Alloy 62 were cast into plates with thickness of 50 mm. 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. 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 four elements at the following indicated atomic percent: Fe (61.0 to 88.0 at. %); Si (0.5 to 9.0 at. %); Mn (0.9 to 19.0 at. %) and optionally B (0.0 at. % to 8.0 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 4.0 at. %); C (0.1 to 4.0 at. %). Impurities may be present include Al, Mo, Nb, S, O, N, P, W, Co, Sn, Zr, Ti, 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 (greater than 60.0 atomic percent) and further including B, Si and Mn. The alloys are capable of being solidified from the melt to form Modal Structure (Structure #1,
Thermal analysis was done on the as-solidified cast sheet samples on a NETZSCH DSC 404F3 PEGASUS V5 system. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) was performed in a range of the temperatures from room temperature to 1425° C. at a heating rate of 10° C./minute with samples protected from oxidation through the use of flowing ultrahigh purity argon. In Table 5, elevated temperature DTA results are shown indicating the melting behavior for the alloys. Note that there were no lower temperature crystallization peaks so metallic glass was not found to be present in the initial castings. As can be seen from the tabulated results in Table 5, the melting occurs in 1 to 4 stages with initial melting observed from ˜1100° C. depending on alloy chemistry. Final melting temperature is >1425° C. in selected alloys. Liquidus temperature for these alloys is out of measurable range and not available (marked as “NA” in the Table 5). Variations in melting behavior may reflect a complex phase formation during chill surface processing of the alloys depending on their chemistry.
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 7.55 g/cm3 to 7.89 g/cm3. The accuracy of this technique is ±0.01 g/cm3.
All cast plates with initial thickness of 3.3 mm (Alloy 1 through Alloy 59) were hot rolled at a temperature that was generally 50° C. below the solidus temperature within a 25° C. range. During the hot rolling step, Dynamic Nanophase Refinement (Mechanism #0,
The tensile specimens were cut from the hot rolled and heat treated sheets using 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; the load cell is attached to the top fixture. In Table 8, a summary of the tensile test results including, yield stress, ultimate tensile strength, and total elongation are shown for the hot rolled sheets after heat treatment. The mechanical characteristic values depend on alloy chemistry and processing condition as will be discussed herein. As can be seen the ultimate tensile strength values vary from 431 to 1612 MPa. The tensile elongation varies from 2.4 to 64.7%. Yield stress is measured in a range from 212 MPa to 966 MPa. During tensile testing, the samples exhibiting Structure #2 (Nanomodal Structure) undergo Mechanism #2 (Dynamic Nanophase Strengthening), to form Structure #3 (High Strength Nanomodal Structure).
All cast plates with initial thickness of 50 mm (Alloy 60 through 62) 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 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%. A list of specific hot rolling parameters used for all alloys is available in Table 9.
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. Examples of specific cold rolling parameters used for the alloys are shown in Table 10.
After hot and cold rolling, tensile specimens were cut via EDM. Part of the samples from each alloy were tested in tension. Tensile properties of the alloys after hot rolling and subsequent cold rolling are listed in Table 11. The ultimate tensile strength values 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. This corresponds to Structure 3 in
Part of cold rolled samples were heat treated at the parameters specified in Table 12. 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 (Table 12) are listed in Table 13. The ultimate tensile strength values 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. This corresponds to Structure 2 in
Plate casting with different thicknesses in a range from 5 to 50 mm using an Indutherm VTC 800 V caster was used to mimic the Stage 1 of the Thin Slab Process (
All cast plates are subjected to hot rolling using a Fenn Model 061 Rolling Mill and a Lucifer 7-R24 Atmosphere Controlled Box Furnace that replicates Stage 2 of the Thin Slab Process with cooling down in air mimicking Stage 3 of the Thin Slab Process (
Sheet samples produced by multi-pass hot rolling of cast plates were the subject for further treatments (heat treatment, cold rolling, etc.) as described in the Case Examples herein mimicking sheet post-processing after Thin Slab Production depending on property and performance requirements for different applications. Close modeling of the Slab Casting process and post-processing methods allow prediction of structural development in the steel alloys herein at each step of the processing and identifies the mechanisms which will lead to production of sheet steel with advanced property combinations.
Using commercial purity feedstock, charges of different masses were weighed out for Alloy 1, Alloy 8, and Alloy 16 according to the atomic ratios provided in Table 4. The charges were then placed into the crucible of an Indutherm VTC 800 V Tilt Vacuum Caster. The feedstock was melted using RF induction and then poured into a copper die designed for casting plates with 50 mm thickness which is in a range for the Thin Slab Casting process (typically 20 to 150 mm). Cast plates from each alloy were heat treated at different parameters listed in Table 15.
Tensile specimens were cut from the as-cast and heat treated plates using a Brother HS-3100 wire electrical discharge machining (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 rigid and the top fixture moving with the load cell attached to the top fixture. A video extensometer was utilized for strain measurements.
Tensile properties of the alloys in the as-cast and heat treated conditions are plotted in
To compare the change in the microstructure caused by heat treatment, samples in as-cast and heat treated states were examined by SEM. To make SEM specimens, the cross-sections of the plate 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 the plate samples from Alloy 1, Alloy 8, and Alloy 16 in the as-cast and heat treated states were examined by scanning electron microscopy (SEM) using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
Thus, Static Nanophase Refinement occurring through elevated temperature heat treatment is found to be relatively ineffective in samples cast at high thickness/reduced cooling rates. The range where Static Nanophase Refinement will not be effective will be dependent on the specific alloy chemistry and size of the dendrites in the Modal Structure but generally occurs at casting thickness greater than or equal to 2.0 mm and cooling rates less than or equal to 250 K/s.
Plate casting with different thicknesses in a range from 1.8 mm to 20 mm was done for the Alloy 58 and Alloy 59 listed in Table 4. Thin plates with as-cast thickness of 1.8 mm were cast in a Pressure Vacuum Caster (PVC). Using commercial purity feedstock, charges of 35 g 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. Individually, the ingots were disc-shaped, with a diameter of ˜30 mm and a thickness of ˜9.5 mm at the thickest point. The resulting ingots were then placed in a PVC chamber, melted using RF induction and then ejected into a copper die designed for casting 3 by 4 inches plates with thickness of 1.8 mm.
Casting of plates with thickness from 5 to 20 mm was done by using an Indutherm VTC 800 V Tilt Vacuum Caster. Using commercial purity feedstock, charges of different masses were weighed out for particular alloys according to the atomic ratios provided in Table 4. The charges were then placed into the crucible of the caster. The feedstock was melted using RF induction and then poured into a copper die designed for casting plates with dimensions described in Table 16.
Each plate from each alloy 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 the specified time of 1 hour for these studies. Note that the HIP cycle was used as in-situ heat treatment and a method to remove some of the casting defects to mimic hot rolling step at slab casting. HIP cycle parameters are listed in Table 17. After HIP cycle, the plates from both alloys were heat treated in a box furnace at 900° C. for 1 hr.
The tensile specimens were cut from the plates in as-HIPed state as well as after HIP cycle and heat treatment using 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. To compare the microstructure change by HIP cycle and heat treatment, samples in the as-cast, HIPed and heat treated states were examined by SEM using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc. To make SEM specimens, the cross-sections of the plate 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.
Tensile properties of the plates from both alloys after HIP cycle are shown in
Examples of microstructures in the plates for Alloy 59 in the as-cast state and after HIP cycle are shown in
This Case Example demonstrates that although HIP cycle at high temperature and additional heat treatment may induce some level of grain refinement within the matrix phase, Static Nanophase Refinement is generally ineffective. Additionally only partial spheroidization of the boundary boride phase can be seen after HIP cycle with complex boride phases localized along the matrix grain boundaries.
Plates with different thicknesses in a range from 5 mm to 20 mm were cast from Alloy 1 and Alloy 2 using an Indutherm VTC 800 V Tilt Vacuum Caster. Using commercial purity feedstock, charges of different masses were weighed out for particular alloys according to the atomic ratios provided in Table 4. The charges were then placed into the crucible of the caster. The feedstock was melted using RF induction and then poured into a copper die designed for casting plates with dimensions described in Table 15. Each plate from each alloy was subjected to Hot Rolling using a Fenn Model 061 Rolling Mill and a Lucifer 7-R24 Atmosphere Controlled Box Furnace. The plates were placed in a furnace pre-heated to 1140° C. for 60 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 during Stage 2 at the Thin Slab Process (
Tensile specimens were cut from the plates after hot rolling using 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. To compare the microstructure in the plates with initial different thicknesses before and after hot rolling, SEM analysis was done on selected samples using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc. To make SEM specimens, the cross-sections of the plate samples from Alloy 1 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.
Tensile properties of the plates from Alloy 1 and Alloy 2 that were cast at different thicknesses and hot-rolled are shown in
This Case Example demonstrates that formation of Homogenized Modal Structure (Structure #1a,
Plate casting with 50 mm thickness from Alloy 1 and Alloy 2 was done using an Indutherm VTC 800 V Tilt Vacuum Caster in order to mimic the Stage 1 of the Thin Slab Process (
Tensile specimens were cut from the rolled and heat treated sheets from Alloy 1 and Alloy 2 using a Brother HS-3100 wire electrical discharge machining (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 rigid and the top fixture moving with the load cell attached to the top fixture. A non-contact video extensometer was utilized for strain measurements.
Tensile properties for Alloy 1 and Alloy 2 sheet after hot rolling and heat treatment at different parameters are plotted in
This Case Example demonstrates that advanced property combinations can be achieved in the alloys herein when cast at 50 mm thickness and undergo Dynamic Nanophase Refinement (Mechanism #0,
Plate casting with 50 mm thickness from Alloy 1 and Alloy 2 was done using an Indutherm VTC 800 V Tilt Vacuum Caster in order to mimic the Stage 1 of the Thin Slab Process (
To evaluate the microstructure in the plates after hot rolling, SEM analysis was done on plate samples from both alloys using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc. To make SEM specimens, the cross-sections of the plate samples from Alloy 1 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. SEM images of the microstructure in Alloy 1 and Alloy 2 plates with as-cast thickness of 50 mm after hot rolling with 96% reduction are shown in
To mimic possible post-processing of the sheet produced by Thick Slab or Thin Slab Process, additional cold rolling with 39% reduction was applied with subsequent heat treatment. Rolled sheet from Alloy 1 was heat treated at 950° C. for 6 hrs and rolled sheet from Alloy 2 was heat treated at 1150° C. for 2 hrs. The tensile specimens were cut from the sheets from Alloy 1 and Alloy 2 using a Brother HS-3100 wire electrical discharge machining (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 rigid and the top fixture moving with the load cell attached to the top fixture. A non-contact video extensometer was utilized for strain measurements.
Tensile properties for Alloy 1 and Alloy 2, in the hot rolled, hot rolled with subsequent cold rolling, and hot rolled with subsequent cold rolling and heat treatment conditions are plotted in
This Case Example demonstrates that advanced property combinations can be achieved in the alloys herein when cast at 50 mm thickness and undergo Dynamic Nanophase Refinement (Mechanism #0,
Plates were cast with different thicknesses in a range from 5 to 50 mm using an Indutherm VTC 800 V caster. Using commercial purity feedstock, charges of different masses were weighed out for particular alloys according to the atomic ratios provided in Table 4. The charges for Alloy 1 and Alloy 2 according to the atomic ratios provided in Table 4 were then placed into the crucible of an Indutherm VTC 800 V Tilt Vacuum Caster. The feedstock was melted using RF induction and then poured into a copper die designed for casting plates with dimensions described in Table 13. All plates from each alloy were subjected to hot rolling using a Fenn Model 061 Rolling Mill and a Lucifer 7-R24 Atmosphere Controlled Box Furnace. The plates were placed in a furnace pre-heated to 1140° C. for 60 minutes prior to the start of rolling. The plates were then repeatedly rolled down to 1.2 to 1.4 mm thickness. To mimic possible post-processing of the sheet produced by the Thin Slab Process, additional cold rolling with 39% reduction was applied to hot rolled plates with subsequent heat treatment at 1150° C. for 2 hrs.
The tensile specimens were cut from the rolled and heat treated sheets from Alloy 1 and Alloy 2 using a Brother HS-3100 wire electrical discharge machining (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 rigid and the top fixture moving with the load cell attached to the top fixture. Video extensometer was utilized for strain measurements. Tensile data for both alloys are plotted in
This Case Example demonstrates that Homogenized Modal Structure (Structure #1a,
Plates with thicknesses of 20 mm were cast from Alloy 2 using an Indutherm VTC 800 V Tilt Vacuum Caster. Using commercial purity feedstock, charges of different masses were weighed out for particular alloy according to the atomic ratios provided in Table 4. The charges were then placed into the crucible of the caster. The feedstock was melted using RF induction and then poured into a copper die designed for casting plates with thickness of 20 mm. Cast plate was subjected to hot rolling using a Fenn Model 061 Rolling Mill and a Lucifer 7-R24 Atmosphere Controlled Box Furnace. The plates were placed in a furnace pre-heated to 1140° C. for 60 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 during Stage 2 at the Thin Slab Process (
To compare the microstructure change by heat treatment, samples after hot rolling and samples after additional 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 2 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 the importance of Nanomodal Structure formation (Structure #2,
Using commercial purity feedstock, charges of different masses were weighed out for Alloy 8 according to the atomic ratios provided in Table 4. The elemental constituents were weighed and charges were cast at 50 mm thickness using a Indutherm VTC 800 V Tilt Vacuum Caster. The feedstock was melted using RF induction and then poured into a water cooled copper die. The cast plates were subjected to hot rolling using a Fenn Model 061 Rolling Mill and a Lucifer 7-R24 Atmosphere Controlled Box Furnace. The samples were hot rolled to approximately 96% reduction in thickness via several rolling passes following a 40 minute soak at 50° C. below each alloy's solidus temperature, mimicking Stage 2 of Thin Slab Production. Between rolling passes, furnace holds of approximately 3 minutes were used to maintain hot rolling temperatures within the slab. Hot rolled sheet was heat treated in inert atmosphere according to the heat treatment schedule in Table 20.
Tensile specimens were cut from the rolled and heat treated sheets from Alloy 8 using a Brother HS-3100 wire electrical discharge machining (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 rigid and the top fixture moving with the load cell attached to the top fixture. Video extensometer was utilized for strain measurements. Tensile data for Alloy 8 after heat treatment at different conditions are plotted in
Alloy 8 that has been heat treated at the HT3 condition (Table 19) possess tensile elongation of >15% with tensile strength approximately 1300 MPa.
This Case Example demonstrates that better properties in Alloy 8 sheet are achieved after additional hot rolling cycles and heat treatment for longer time (HT1, Table 19) or higher temperature (HT3, Table 19) when more complete transformation into the Nanomodal Structure (Structure #2,
Using commercial purity feedstock, charges of different masses were weighed out for Alloy 16 according to the atomic ratios provided in Table 4. The elemental constituents were weighed and charges were cast at 50 mm thickness using an Indutherm VTC 800 V Tilt Vacuum Caster. The feedstock was melted using RF induction and then poured into a water cooled copper die. Slab casting corresponds to Stage 1 of Thin Slab Production. Cast plates were subjected to hot rolling using a Fenn Model 061 Rolling Mill and a Lucifer 7-R24 Atmosphere Controlled Box Furnace. The samples were hot rolled to ˜96% reduction in thickness via several rolling passes (10 total) following a 40 minute soak at 50° C. below Alloy 16's solidus temperature, mimicking Stage 2 of Thin Slab Production. Between rolling passes, furnace holds of approximately 3 minutes were used to maintain hot rolling temperatures within the slab. During the hot rolling steps, Dynamic Nanophase Refinement (Mechanism #0) was activated. Hot rolled sheet was heat treated in inert atmosphere according to the heat treatment schedule in Table 21.
Tensile specimens were cut from the rolled and heat treated sheets from Alloy 16 using a Brother HS-3100 wire electrical discharge machining (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 rigid and the top fixture moving with the load cell attached to the top fixture. Video extensometer was utilized for strain measurements. Tensile data for Alloy 16 after heat treatment at different conditions are plotted in
This Case Example demonstrates that better properties can be achieved in Alloy 16 hot rolled sheet after heat treatment at highest temperature (HT6, Table 20) that seems to correspond to most optimal conditions for complete transformation through Static Nanophase Refinement (Mechanism #1,
Using commercial purity feedstock, charges of different masses were weighed out for Alloy 24 according to the atomic ratios provided in Table 4. The elemental constituents were weighed and charges were cast at 50 mm thickness using a Indutherm VTC 800 V Tilt Vacuum Caster. The feedstock was melted using RF induction and then poured into a water cooled copper die. Slab casting corresponds to Stage 1 of Thin Slab Production. Cast plates were subjected to hot rolling using a Fenn Model 061 Rolling Mill and a Lucifer 7-R24 Atmosphere Controlled Box Furnace. The samples were hot rolled to ˜96% reduction in thickness via several rolling passes following a 40 minute soak at 50° C. below the alloy's solidus temperature, mimicking Stage 2 of Thin Slab Production. Between rolling passes, furnace holds of approximately 3 minutes were used to maintain hot rolling temperatures within the slab. Hot rolled sheet was heat treated in inert atmosphere according to the heat treatment schedule in Table 22.
Tensile specimens were cut from the rolled and heat treated sheets from Alloy 24 using a Brother HS-3100 wire electrical discharge machining (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 rigid and the top fixture moving with the load cell attached to the top fixture. Video extensometer was utilized for strain measurements. Tensile data for Alloy 24 after heat treatment at different conditions are plotted in
This Case Example demonstrates that heat treatment at all three conditions resulted in strength decrease with increasing ductility suggesting that Nanomodal Structure (Structure #2,
A 50 mm thick Alloy 1 plate was hot rolled at 1150° C. with a two-step reduction by 85.2% and 73.9% respectively and then heat treated at 950° C. for 6 hrs. Tensile tests were conducted on samples after the heat treatment. Microstructures of samples before and after the uniaxial deformation were studied by transmission electron microscopy (TEM). TEM specimens were cut from the grip section and tensile gage of test specimens, representing the states before and after tensile deformation respectively. TEM specimen preparation procedure includes cutting, thinning, 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 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 usually 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 image of the microstructure in the Alloy 1 plate after hot rolling and heat treatment before deformation is shown in
This Case Example illustrates High Strength Nanomodal Structure formation (Structure #3,
Samples of 50 mm thick Alloy 8 plate were hot rolled at 1150° C. and heat treated at 950° C. for 6 hrs. Tensile tests were conducted on samples after the heat treatment. Microstructures of samples before and after the tensile deformation were studied by transmission electron microscopy (TEM). TEM specimens were cut from the grip section and tensile gage of test specimens, representing the states before and after tensile deformation respectively. TEM specimen preparation procedure includes cutting, thinning, electropolishing. First, samples were cut with electric discharge machine (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 usually 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 image of the microstructure in the Alloy 8 plate after hot rolling and heat treatment before deformation is shown in
After tensile deformation, further microstructure refinement may be seen in the sample, and nano-size precipitate formation in Alloy 8 was found. As shown in
This Case Example illustrates High Strength Nanomodal Structure formation (Structure #3,
Samples of 50 mm thick Alloy 16 plate were hot rolled at 1150° C. and heat treated at 1150° C. for 2 hrs. Tensile tests were conducted on samples after the heat treatment. Microstructures of samples before and after the tensile deformation were studied by transmission electron microscopy (TEM). TEM specimens were cut from the grip section and tensile gage of test specimens, representing the states before and after tensile deformation respectively. TEM specimen preparation procedure includes cutting, thinning, 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 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 usually 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 image of the Alloy 16 slab sample before deformation is shown in
After tensile deformation, substantial microstructure refinement is observed in the sample.
This Case Example illustrates High Strength Nanomodal Structure formation (Structure #3,
Plates with 50 mm thickness from Alloy 32 and Alloy 42 were cast using a Indutherm VTC 800 V Tilt Vacuum Caster was utilized to mimic the Stage 1 of the Thin Slab Process (
The tensile specimens were cut from the rolled and heat treated sheets from Alloy 32 and Alloy 42 using a Brother HS-3100 wire electrical discharge machining (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 rigid and the top fixture moving with the load cell attached to the top fixture. A video extensometer was utilized for strain measurements.
Tensile properties for both alloys are plotted in
This Case Example demonstrated properties in Alloy 32 and Alloy 42 plates cast at 50 mm thickness and undergoing hot rolling. High ductility in these alloys suggests that the Homogenized Modal Structure of Type 1 (Table 3) was formed during hot rolling.
The structural evolution in Alloy 24 plate initially cast at 50 mm thickness was studied by TEM. The casting was done using a Indutherm VTC 800 V Tilt Vacuum Caster, and then the slab was hot rolled to 2 mm thick sheet at 1100° C. To study the structural evolution, samples from Alloy 24 in the as-cast and hot rolled conditions 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 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 a 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 microstructure of as-cast plate is shown in
After hot rolling, the boride phase was broken up into small particles and is well scattered in the matrix indicating structural homogenization through Dynamic Nanophase Refinement (Mechanism #0,
This Case Example demonstrated structural development in Alloy 24 plate cast at 50 mm thickness and undergoing hot rolling. Microstructural evolution is following a pathway towards desired structure formation illustrated in
Elastic Modulus was measured for selected alloys listed in Table 22. Each alloy used was cast into a plate with thickness of 50 mm. Using a high temperature inert gas furnace the material was brought to the desired temperature, depending on alloy solidus temperature, prior to hot rolling. Initial hot rolling reduced the material thickness by approximately 85%. The oxide layer was removed from the hot rolled material using abrasive media. The center was sectioned from the resulting slab and hot rolled approximately an additional 75%. After removing the final oxide layer ASTM E8 subsize tensile samples were cut from center of the resulting material using wire electrical discharge machining (EDM). 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. Elastic modulus data in Table 23 is reported as an average value of 5 separate measurements. Modulus values vary in a range from 190 to 210 GPa typical for commercial steels and depend on alloy chemistry and thermo-mechanical treatment.
This Case Example demonstrates that modulus values of the alloy herein vary in a range from 190 to 210 GPa which is typical for commercial steels and depend on alloy chemistry and thermo-mechanical treatment.
Using commercial purity feedstock, charges of different masses were weighed out for selected alloys according to the atomic ratios provided in Table 4. The elemental constituents were weighed on an analytical balance and the charges were cast at 50 mm thickness using a Indutherm VTC 800 V Tilt Vacuum Caster. The feedstock was melted using RF induction and then poured into a water cooled copper die forming a cast plate. Plate casting corresponds to Stage 1 of Thin Slab Production (
In the center of the cast plate was a shrinkage funnel that was created by the solidification of the last amount of liquid metal. A schematic of the cross section through the center of the plate is shown in
Two thin sections that were ˜4 mm thick were cut using wire electrical discharge machining (EDM) one from the top and the other from bottom of the cast plate. Small samples from the center of the bottom thin section (marked “B” in
The results of the chemical analysis are shown in
This Case Example demonstrates that in spite of the cast plate thickness of 50 mm, there was no macrosegregation detected in the cast plates from alloys herein.
Tensile properties of selected alloys from Table 4 were compared with tensile properties of existing steel grades. The selected alloys and corresponding parameters are listed in Table 24. Tensile stress-strain curves are compared to that of existing Dual Phase (DP) steels (
This case Example demonstrates that the alloys disclosed here have relatively superior mechanical properties as compared to existing advanced high strength (AHSS) steel grades with. Ductility of 20% and above demonstrated by selected alloys provides cold formability of the sheet material and make it applicable to many processes such as for example cold stamping of a relatively complex part.
Plate casting with 50 mm thickness from Alloy 1, Alloy 8, Alloy 16, Alloy 24, Alloy 26, Alloy 32, and Alloy 42 was done using an Indutherm VTC 800 V Tilt Vacuum Caster in order to mimic the Stage 1 of the Thin Slab Process (
Rolled sheet from each alloy was heat treated at different conditions specified in Table 7. The tensile specimens were cut from the sheets using a Brother HS-3100 wire electrical discharge machining (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 rigid and the top fixture moving with the load cell attached to the top fixture. A non-contact video extensometer was utilized for strain measurements.
Tensile properties for Alloy 1, Alloy 8, Alloy 16, Alloy 24, Alloy 26, Alloy 32, and Alloy 42 after hot rolling and subsequent heat treatment (Table 25) are plotted in
This Case Example demonstrates that same level of properties achieved in the alloys herein when casting thickness increased from 3.3 mm to 50 mm confirming that mechanisms in alloys herein follows the pathway illustrated in
The chemical composition of the boron-free alloys herein (Alloy 63 through Alloy 74) is listed 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 which is in the thickness range for the Thin Slab Casting process and 75 mm×100 mm in size.
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 26, melting occurs in one stage except in Alloy 65 with melting in two stages. Initial melting recorded from minimum at ˜1278° C. and depends on Alloy chemistry. Maximum final melting temperature recorded at 1450° C.
The 50 mm thick laboratory slab from each alloy was subjected to hot rolling at the temperature of 1250° C. except that from Alloy 68 which was rolled at 1250° C. 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 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 80% to 88% total reduction to a thickness of between 6 mm and 9.5 mm. Following the first campaign of hot rolling, a section of sheet between 130 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 27.
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 28 and was found to vary from 7.64 to 7.80 g/cm3. Experimental results have revealed that the accuracy of this technique is ±0.01 g/cm3.
The fully hot-rolled sheet was then subjected to 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 29.
After hot and cold rolling, tensile specimens were cut via EDM. The resultant samples were heat treated at the parameters specified in Table 30. Hydrogen heat treatments were conducted in a CAMCo G1200-ATM sealed atmosphere furnace. Samples were loaded at room temperature and were heated to the target dwell temperature at 1200° C./hour. Dwells were conducted under atmospheres listed in Table 30. Samples were cooled under furnace control in an argon atmosphere. Hydrogen-free 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 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 31. The ultimate tensile strength values may vary from 947 to 1329 MPa with tensile elongation from 20.5 to 55.4%. The yield stress is in a range from 267 to 520 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry and hot rolling conditions. An example stress-strain curve for Alloy 63 in as hot rolled state is shown in
Tensile properties of selected alloys after hot rolling and subsequent cold rolling are listed in Table 32 which represent Structure #3 or the High Strength Nanomodal Structure. The ultimate tensile strength values may vary from 1402 to 1766 MPa with tensile elongation from 9.7 to 29.1%. The yield stress is in a range from 913 to 1278 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry and processing conditions.
Tensile properties of the hot rolled sheets after hot rolling with subsequent heat treatment at different parameters (Table 30) are listed in Table 33. The ultimate tensile strength values may vary from 669 to 1352 MPa with tensile elongation from 15.9% to 78.1%. The yield stress is in a range from 217 to 621 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry and processing conditions.
This Case Example demonstrates that mechanisms in boron-free alloys follow the pathway illustrated in
Plate with 50 mm thickness from Alloy 65 was cast in an Indutherm VTC800V vacuum tilt casting machine. Alloy of designated composition was 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. Weighed out Alloy charge was 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. Alloy charge was 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. Melt was then poured into a water-cooled copper die to form laboratory cast slab of approximately 50 mm thick which is in the thickness range for the Thin Slab Casting process and 75 mm×100 mm in size.
The 50 mm thick laboratory slab from the Alloy 65 was subjected to hot rolling at the temperature of 1250° C. with a total reduction of 97%. The fully hot-rolled sheet was then subjected to cold rolling in multiple passes down to thickness of 1.2 mm. Cold rolled sheet was heat treated at 850° C. for 5 minutes that mimic in-line annealing at commercial sheet production. To make SEM specimens, the cross-sections of the sheet sample in as-cast state, after hot rolling, and after cold rolling with subsequent heat treatment 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 samples from Alloy 65 were examined by scanning electron microscopy (SEM) using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
This Case Example demonstrates structural development in boron-free alloys is similar to that for alloys containing boron (
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/896,594, filed on Oct. 28, 2013, which is fully incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61896594 | Oct 2013 | US |
