Patent Application
20210238703
This disclosure relates to methods for mechanical property improvement in a metallic alloy that has undergone one or more mechanical property losses as a consequence of shearing, such as in the formation of a sheared edge portion or a punched hole. More specifically, methods are disclosed that provide the ability to improve mechanical properties of metallic alloys that have been formed with one or more sheared edges which may otherwise serve as a limiting factor for industrial applications.
From ancient tools to modem skyscrapers and automobiles, steel has driven human innovation for hundreds of years. Abundant in the Earth's crust, iron and its associated alloys have provided humanity with solutions to many daunting developmental barriers. From humble beginnings, steel development has progressed considerably within the past two centuries, with new varieties of steel becoming available every few years. These steel alloys can be broken up into three classes based upon measured properties, in particular maximum tensile strain and tensile stress prior to failure. These three classes are: Low Strength Steels (LSS), High Strength Steels (HSS), and Advanced High Strength Steels (AHSS). Low Strength Steels (LSS) are generally classified as exhibiting ultimate tensile strengths less than 270 MPa and include such types as interstitial free and mild steels. High-Strength Steels (HSS) are classified as exhibiting ultimate tensile strengths from 270 to 700 MPa and include such types as high strength low alloy, high strength interstitial free and bake hardenable steels. Advanced High-Strength Steels (AHSS) steels are classified by ultimate tensile strengths greater than 700 MPa and include such types as Martensitic steels (MS), Dual Phase (DP) steels, Transformation Induced Plasticity (TRIP) steels, and Complex Phase (CP) steels. As the strength level increases the trend in maximum tensile elongation (ductility) of the steel is negative, with decreasing elongation at high ultimate tensile strengths. For example, tensile elongation of LSS, HSS and AHSS ranges from 25% to 55%, 10% to 45%, and 4% to 30%, respectively.
Production of steel continues to increase, with a current US production around 100 million tons per year with an estimated value of S75 billion. Steel utilization in vehicles is also high, with advanced high strength steels (AHSS) currently at 17% and forecast to grow by 300% in the coming years [American Iron and Steel Institute. (2013). Profile 2013. Washington, D.C.]. With current market trends and governmental regulations pushing towards higher efficiency in vehicles, AHSS are increasingly being pursued for their ability to provide high strength to mass ratio. The high strength of AHSS allows for a designer to reduce the thickness of a finished part while still maintaining comparable or improved mechanical properties. In reducing the thickness of a part, less mass is needed to attain the same or better mechanical properties for the vehicle thereby improving vehicle fuel efficiency. This allows the designer to improve the fuel efficiency of a vehicle while not compromising on safety.
One key attribute for next generation steels is formability. Formability is the ability of a material to be made into a particular geometry without cracking, rupturing or otherwise undergoing failure. High formability steel provides benefit to a part designer by allowing for the creation of more complex part geometries allowing for reduction in weight. Formability may be further broken into two distinct forms: edge formability and bulk formability. Edge formability is the ability for an edge to be formed into a certain shape. Edges on materials are created through a variety of methods in industrial processes, including but not limited to punching, shearing, piercing, stamping, perforating, cutting, or cropping. Furthermore, the devices used to create these edges are as diverse as the methods, including but not limited to various types of mechanical presses, hydraulic presses, and/or electromagnetic presses. Depending upon the application and material undergoing the operation, the range of speeds for edge creation is also widely varying, with speeds as low as 0.25 mm/s and as high as 3700 mm/s. The wide variety of edge forming methods, devices, and speeds results in a myriad of different edge conditions in use commercially today.
Edges, being free surfaces, are dominated by defects such as cracks or structural changes in the sheet resulting from the creation of the sheet edge. These defects adversely affect the edge formability during forming operations, leading to a decrease in effective ductility at the edge. Bulk formability on the other hand is dominated by the intrinsic ductility, structure, and associated stress state of the metal during the forming operation. Bulk formability is affected primarily by available deformation mechanisms such as dislocations, twinning, and phase transformations. Bulk formability is maximized when these available deformation mechanisms are saturated within the material, with improved bulk formability resulting from an increased number and availability of these mechanisms.
Edge formability can be measured through hole expansion measurements, whereby a hole is made in a sheet and that hole is expanded by means of a conical punch. Previous studies have shown that conventional AHSS materials suffer from reduced edge formability compared with other LSS and HSS when measured by hole expansion [M. S. Billur, T. Altan, “Challenges in forming advanced high strength steels”, Proceedings of New Developments in Sheet Metal Forming, pp. 285-304, 2012]. For example, Dual Phase (DP) steels with ultimate tensile strength of 780 MPa achieve less than 20% hole expansion, whereas Interstitial Free steels (IF) with ultimate tensile strength of approximately 400 MPa achieve around 100% hole expansion ratio. This reduced edge formability complicates adoption of AHSS in automotive applications, despite possessing desirable bulk formability.
The present invention provides a cold rolled steel sheet product comprising Fe and at least four alloying elements selected from Si, Mn, B, Cr, Ni, Cu and C, wherein the steel sheet product includes a sheared edge, has an ultimate tensile strength of at least 799 MPa, a total elongation of at least 6.6 percent, and a hole expansion ratio greater than 20.
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.
Alloy 1 with holes prepared by different methods; a) Punched hole, b) EDM cut hole, c) Milled hole, and d) Laser cut hole.
The steel alloys herein undergo a unique pathway of structural formation through specific mechanisms as illustrated in
Steel alloys herein with the Modal Structure (Structure #1,
The Nanomodal Structure (Structure #2,
When steel alloys herein with the Nanomodal Structure (Structure #2,
The High Strength Nanomodal Structure (Structure #3,
Steel alloys herein with the Recrystallized Modal Structure (Structure #4,
Steel alloys herein with the Refined High Strength Nanomodal Structure (Structure #5,
The structures and enabling mechanisms for the steel alloys herein are applicable to commercial production using existing process flows. See
The formation of Modal Structure (Structure #1) in steel alloys herein occurs during alloy solidification. The Modal Structure may be preferably formed by heating the alloys herein at temperatures in the range of above their melting point and in a range of 1100° C. to 2000° C. and cooling below the melting temperature of the alloy, which corresponds to preferably cooling in the range of 1×103 to 1×10−3 K/s. The as-cast thickness will be dependent on the production method with Thin Slab Casting typically in the range of 20 to 150 mm in thickness and Thick Slab Casting typically in the range of 150 to 500 mm in thickness. Accordingly, as cast thickness may fall in the range of 20 to 500 mm, and at all values therein, in 1 nun increments. Accordingly, as cast thickness may be 21 mm, 22 mm, 23 mm, etc., up to 500 mm.
Hot rolling of solidified slabs from the alloys is the next processing step with production either of transfer bars in the case of Thick Slab Casting or coils in the case of Thin Slab Casting. During this process, the Modal Structure transforms in a continuous fashion into a partial and then fully Homogenized Modal Structure (Structure #1a) through Nanophase Refinement (Mechanism #1). Once homogenization and resulting refinement is completed, the Nanomodal Structure (Structure #2) forms. The resulting hot band coils which are a product of the hot rolling process is typically in the range of 1 to 20 mm in thickness.
Cold rolling is a widely used method for sheet production that is utilized to achieve targeted thickness for particular applications. For AHSS, thinner gauges are usually targeted in the range of 0.4 to 2 mm. To achieve the finer gauge thicknesses, cold rolling can be applied through multiple passes with or without intermediate annealing between passes. Typical reduction per pass is 5 to 70% depending on the material properties and equipment capability. The number of passes before the intermediate annealing also depends on materials properties and level of strain hardening during cold deformation. For the steel alloys herein, the cold rolling will trigger Dynamic Nanophase Strengthening (Mechanism #2) leading to extensive strain hardening of the resultant sheet and to the formation of the High Strength Nanomodal Structure (Structure #3). The properties of the cold rolled sheet from alloys herein will depend on the alloy chemistry and can be controlled by the cold rolling reduction to yield a fully cold rolled (i.e. hard) product or can be done to yield a range of properties (i.e. ¼, ½, ¾ hard etc.). Depending on the specific process flow, especially starting thickness and the amount of hot rolling gauge reduction, often annealing is needed to recover the ductility of the material to allow for additional cold rolling gauge reduction. Intermediate coils can be annealed by utilizing conventional methods such as batch annealing or continuous annealing lines. The cold deformed High Strength Nanomodal Structure (Structure #3) for the steel alloys herein will undergo Recrystallization (Mechanism #3) during annealing leading to the formation of the Recrystallized Modal Structure (Structure #4). At this stage, the recrystallized coils can be a final product with advanced property combination depending on the alloy chemistry and targeted markets. In a case when even thinner gauges of the sheet are required, recrystallized coils can be subjected to further cold rolling to achieve targeted thickness that can be realized by one or multiple cycles of cold rolling/annealing. Additional cold deformation of the sheet from alloys herein with Recrystallized Modal Structure (Structure 44) leads to structural transformation into Refined High Strength Nanomodal Structure (Structure #5) through Nanophase Refinement and Strengthening (Mechanism #4). As a result, fully hard coils with final gauge and Refined High Strength Nanomodal Structure (Structure #5) can be formed or, in the case of annealing as a last step in the cycle, coils of the sheet with final gauge and Recrystallized Modal Structure (Structure #4) can also be produced. When coils of recrystallized sheet from alloys herein utilized for finished part production by any type of cold deformation such as cold stamping, hydroforming, roll forming etc., Refined High Strength Nanomodal Structure (Structure #5) will be present in the final product/parts. The final products may be in many different forms including sheet, plate, strips, pipes, and tubes and a myriad of complex parts made through various metalworking processes.
The cyclic nature of these phase transformations going from Recrystallized Modal Structure (Structure #4) to Refined High Strength Nanomodal Structure (Structure #5) and then back to Recrystallized Modal Structure (Structure #4) is one of the unique phenomenon and features of steel alloys herein. As described earlier, this cyclic feature is applicable during commercial manufacturing of the sheet, especially for AHSS where thinner gauge thicknesses are required (e.g. thickness in the range of 0.2 to 25 mm). Furthermore, these reversibility mechanisms are applicable for the widespread industrial usage of the steel alloys herein. While exhibiting exceptional combinations of bulk sheet formability as is demonstrated by the tensile and bend properties in this application for the steel alloys herein, the unique cycle feature of the phase transformations is enabling for edge formability, which can be a significant limiting factor for other AHSS. Table 1 below provides a summary of the structure and performance features through stressing and heating cycles available through Nanophase Refinement and Strengthening (Mechanism #4). How these structures and mechanisms can be harnessed to produce exceptional combinations of both bulk sheet and edge formability will be subsequently described herein.
The chemical composition of the alloys herein is shown in Table 2 which provides the preferred atomic ratios utilized.
As can be seen from the above, the alloys herein are iron based metal alloys, having greater than or equal to 50 at. % Fe. More preferably, the alloys herein can be described as comprising, consisting essentially of, or consisting of the following elements at the indicated atomic percent: Fe (61.30 to 83.14 at. %); Si (0 to 7.02 at. %); Mn (0 to 15.86 at. %); B (0 to 6.09 at. %); Cr (0 to 18.90 at. %); Ni (0 to 8.68 at. %); Cu (0 to 2.00 at. %); C (0 to 3.72 at. %). In addition, it can be appreciated that the alloys herein are such that they comprise Fe and at least four or more, or five or more, or six or more elements selected from Si, Mn, B, Cr, Ni, Cu or C. Most preferably, the alloys herein are such that they comprise, consist essentially of, or consist of Fe at a level of 50 at. % or greater along with Si, Mn, B, Cr, Ni, Cu and C.
Laboratory processing of the alloys in Table 2 was done to model each step of industrial production but on a much smaller scale. Key steps in this process include the following: casting, tunnel furnace heating, hot rolling, cold rolling, and annealing.
Alloys were weighed out into charges ranging from 3,000 to 3,400 grams using commercially available ferroadditive powders with known chemistry and impurity content according to the atomic ratios in Table 2. Charges were loaded into a zirconia coated silica crucibles which was placed into an Indutherm VTC800V vacuum tilt casting machine. The machine then evacuated the casting and melting chambers and backfilled with argon to atmospheric pressure several times prior to casting to prevent oxidation of the melt. The melt was heated with a 14 kHz RF induction coil until fully molten, approximately 5.25 to 6.5 minutes depending on the alloy composition and charge mass. After the last solids were observed to melt it was allowed to heat for an additional 30 to 45 seconds to provide superheat and ensure melt homogeneity. The casting machine then evacuated the melting and casting chambers, tilted the crucible and poured the melt into a 50 mm thick, 75 to 80 mm wide, and 125 mm deep channel in a water cooled copper die. The melt was allowed to cool under vacuum for 200 seconds before the chamber was filled with argon to atmospheric pressure. Example pictures of laboratory cast slabs from two different alloys are shown in
Prior to hot rolling, laboratory slabs were loaded into a Lucifer EHS3GT-B18 furnace to heat. The furnace set point varies between 1100° C. to 1250° C. depending on alloy melting point. The slabs were allowed to soak for 40 minutes prior to hot rolling to ensure they reach the target temperature. Between hot rolling passes the slabs are returned to the furnace for 4 minutes to allow the slabs to reheat.
Pre-heated slabs were pushed out of the tunnel furnace into a Fenn Model 061 2 high rolling mill. The 50 mm slabs were preferably hot rolled for 5 to 8 passes though the mill before being allowed to air cool. After the initial passes each slab had been reduced between 80 to 85% to a final thickness of between 7.5 and 10 mm. After cooling each resultant sheet was sectioned and the bottom 190 mm was hot rolled for an additional 3 to 4 passes through the mill, further reducing the plate between 72 to 84% to a final thickness of between 1.6 and 2.1 mm. Example pictures of laboratory cast slabs from two different alloys after hot rolling are shown in
After hot rolling resultant sheets were media blasted with aluminum oxide to remove the mill scale and were then cold rolled on a Fenn Model 061 2 high rolling mill. Cold rolling takes multiple passes to reduce the thickness of the sheet to a targeted thickness of typically 1.2 mm. Hot rolled sheets were fed into the mill at steadily decreasing roll gaps until the minimum gap is reached. If the material has not yet hit the gauge target, additional passes at the minimum gap were used until 1.2 mm thickness was achieved. A large number of passes were applied due to limitations of laboratory mill capability. Example pictures of cold rolled sheets from two different alloys are shown in
After cold rolling, tensile specimens were cut from the cold rolled sheet via wire electrical discharge machining (EDM). These specimens were then annealed with different parameters listed in Table 3. Annealing 1a, 1b, 2b were conducted in a Lucifer 7HT-K12 box furnace. Annealing 2a and 3 was conducted in a Camco Model G-ATM-12FL furnace. Specimens which were air normalized were removed from the furnace at the end of the cycle and allowed to cool to room temperature in air. For the furnace cooled specimens, at the end of the annealing the furnace was shut off to allow the sample to cool with the furnace. Note that the heat treatments were selected for demonstration but were not intended to be limiting in scope. High temperature treatments up to just below the melting points for each alloy are possible.
Thermal analysis of the alloys herein was performed on as-solidified cast slabs using a Netzsch Pegasus 404 Differential Scanning calorimeter (DSC). Samples of alloys were loaded into alumina crucibles which were then loaded into the DSC. The DSC then evacuated the chamber and backfilled with argon to atmospheric pressure. A constant purge of argon was then started, and a zirconium getter was installed in the gas flow path to further reduce the amount of oxygen in the system. The samples were heated until completely molten, cooled until completely solidified, then reheated at 10° C./min through melting. Measurements of the solidus, liquidus, and peak temperatures were taken from the second melting in order to ensure a representative measurement of the material in an equilibrium state. In the alloys listed in Table 2, melting occurs in one or multiple stages with initial melting from ˜1111° C. depending on alloy chemistry and final melting temperature up to ˜1476° C. (Table 4). Variations in melting behavior reflect complex phase formation at solidification of the alloys depending on their chemistry.
The density of the alloys was measured on 9 mm thick sections of hot rolled material using the Archimedes method in a specially constructed balance allowing weighing in both air and distilled water. The density of each alloy is tabulated in Table 5 and was found to be in the range from 7.57 to 7.89 g/cm3. The accuracy of this technique is ±0.01 g/cm3.
Tensile properties were measured on an Instron 3369 mechanical testing frame using Instron's Bluehill control software. All tests were conducted at room temperature, with the bottom grip fixed and the top grip set to travel upwards at a rate of 0.012 mm/s. Strain data was collected using Instron's Advanced Video Extensometer. Tensile properties of the alloys listed in Table 2 after annealing with parameters listed in Table 3 are shown below in Table 6 to Table 10. The ultimate tensile strength values may vary from 799 to 1683 MPa with tensile elongation from 6.6 to 86.7%. The yield strength is in a range from 197 to 978 MPa. The mechanical characteristic values in the steel alloys herein will depend on alloy chemistry and processing conditions. The variation in heat treatment additionally illustrates the property variations possible through processing a particular alloy chemistry.
A laboratory slab with thickness of 50 mm was cast from Alloy 1 that was then laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 5 min as described in Main Body section of current application. Microstructure of the alloy was examined at each step of processing by SEM, TEM and x-ray analysis.
For SEM study, the cross section of the slab samples was ground on SiC abrasive papers with reduced grit size, and then polished progressively with diamond media paste down to 1 μm. The final polishing was done with 0.02 gm grit SiO2 solution. Microstructures were examined by SEM using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc. To prepare TEM specimens, the samples were first cut by EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to make foils of 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 completed with electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling usually is 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. X-ray diffraction was done using a PANalytical X'Pert MPD difftactometer with a Cu Kα x-ray tube and operated at 45 kV with a filament current of 40 mA. Scans were run with a step size of 0.01° and from 25° to 95° two-theta with silicon incorporated to adjust for instrument zero angle shift. The resulting scans were then subsequently analyzed using Rietveld analysis using Siroquant software.
Modal Structure was formed in the Alloy 1 slab with 50 mm thickness after solidification. The Modal Structure (Structure #1) is represented by a dendritic structure that is composed of several phases. In
Deformation of the Alloy 1 with the Modal Structure (Structure #1,
Further deformation at ambient temperature (i.e., cold deformation) of the Alloy 1 with the Nanomodal Structure causes transformation into High Strength Nanomodal Structure (Structure #3,
Recrystallization occurs upon heat treatment of the cold deformed Alloy 1 with High Strength Nanomodal Structure (Structure #3,
When the Alloy 1 with Recrystallized Modal Structure (Structure #4,
This Case Example demonstrates that alloys listed in Table 2 including Alloy 1 exhibit a structural development pathway with novel enabling mechanisms illustrated in
Laboratory slab with thickness of 50 mm was cast from Alloy 2 that was then laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described in Main Body section of current application. Microstructure of the alloy was examined at each step of processing by SEM, TEM and x-ray analysis.
For SEM study, the cross section of the slab samples was ground on SiC abrasive papers with reduced grit size, and then polished progressively with diamond media paste down to 1 μm. The final polishing was done with 0.02 μm grit SiO2 solution. Microstructures were examined by SEM using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc. To prepare TEM specimens, the samples were first cut with EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to make foils to ˜60 μm thickness was done by polishing with 9 μm, 3 μm and 1 μm diamond suspension solution respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled with electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling usually is 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. X-ray diffraction was done using a Panalytical X'Pert MPD diffractometer with a Cu Kα x-ray tube and operated at 45 kV with a filament current of 40 mA. Scans were run with a step size of 0.01° and from 25° to 95° two-theta with silicon incorporated to adjust for instrument zero angle shift. The resulting scans were then subsequently analyzed using Rietveld analysis using Siroquant software.
Modal Structure (Structure #1,
Following the flowchart in
Deformation of the Alloy 2 with the Nanomodal Structure but at ambient temperature (i.e., cold deformation) leads to formation of High Strength Nanomodal Structure (Structure #3,
Recrystallization occurs upon annealing of the cold deformed Alloy 2 with High Strength Nanomodal Structure (Structure #3,
Deformation of Recrystallized Modal Structure (Structure #4,
This Case Example demonstrates that alloys listed in Table 2 including Alloy 2 exhibit a structural development pathway with the mechanisms illustrated in
Slabs with thickness of 50 mm were laboratory cast from the alloys listed in Table 21 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described in Main Body section of current application. Tensile properties were measured at each step of processing on an Instron 3369 mechanical testing frame using Instron's Bluehill control software. All tests were conducted at room temperature, with the bottom grip fixed and the top grip set to travel upwards at a rate of 0.012 mm/s. Strain data was collected using Instron's Advanced Video Extensometer.
Alloys were weighed out into charges ranging from 3,000 to 3,400 grams using commercially available ferroadditive powders with known chemistry and impurity content according to the atomic ratios in Table 2. Charges were loaded into zirconia coated silica crucibles which were placed into an Indutherm VTC800V vacuum tilt casting machine. The machine then evacuated the casting and melting chambers and backfilled with argon to atmospheric pressure several times prior to casting to prevent oxidation of the melt. The melt was heated with a 14 kHz RF induction coil until fully molten, approximately 5.25 to 6.5 minutes depending on the alloy composition and charge mass. After the last solids were observed to melt it was allowed to heat for an additional 30 to 45 seconds to provide superheat and ensure melt homogeneity. The casting machine then evacuated the melting and casting chambers and tilted the crucible and poured the melt into a 50 mm thick, 75 to 80 mm wide, and 125 mm deep channel in a water cooled copper die. The melt was allowed to cool under vacuum for 200 seconds before the chamber was filled with argon to atmospheric pressure. Tensile specimens were cut from as-cast slabs by wire EDM and tested in tension. Results of tensile testing are shown in Table 21. As it can be seen, ultimate tensile strength of the alloys herein in as-cast condition varies from 411 to 907 MPa. The tensile elongation varies from 3.7 to 24.4%. Yield strength is measured in a range from 144 to 514 MPa.
Prior to hot rolling, laboratory cast slabs were loaded into a Lucifer EHS3GT-B18 furnace to heat. The furnace set point varies between 1000° C. to 1250° C. depending on alloy melting point. The slabs were allowed to soak for 40 minutes prior to hot rolling to ensure they reach the target temperature. Between hot rolling passes the slabs are returned to the furnace for 4 minutes to allow the slabs to reheat. Pre-heated slabs were pushed out of the tunnel furnace into a Fenn Model 061 2 high rolling mill. The 50 mm casts are hot rolled for 5 to 8 passes through the mill before being allowed to air cool defined as first campaign of hot rolling. After this campaign the slab thickness was reduced between 80.4 to 87.4%. After cooling, the resultant sheet samples were sectioned to 190 mm in length. These sections were hot rolled for an additional 3 passes through the mill with reduction between 73.1 to 79.9% to a final thickness of between 2.1 and 1.6 mm. Detailed information on hot rolling conditions for each alloy herein is provided in Table 22. Tensile specimens were cut from hot rolled sheets by wire EDM and tested in tension. Results of tensile testing are shown in Table 22. After hot rolling, ultimate tensile strength of the alloys herein varies from 921 to 1413 MPa. The tensile elongation varies from 12.0 to 77.7%. Yield strength is measured in a range from 264 to 574 MPa. See, Structure 2 in
After hot rolling, resultant sheets were media blasted with aluminum oxide to remove the mill scale and were then cold roiled on a Fenn Model 061 2 high rolling mill. Cold rolling takes multiple passes to reduce the thickness of the sheet to targeted thickness, generally 1.2 mm. Hot rolled sheets were fed into the mill at steadily decreasing roll gaps until the minimum gap is reached. If the material has not yet hit the gauge target, additional passes at the minimum gap were used until the targeted thickness was reached. Cold rolling conditions with the number of passes for each alloy herein are listed in Table 23. Tensile specimens were cut from cold rolled sheets by wire EDM and tested in tension. Results of tensile testing are shown in Table 23. Cold rolling leads to significant strengthening with ultimate tensile strength in the range from 1356 to 1831 MPa. The tensile elongation of the alloys herein in cold rolled state varies from 1.6 to 32.1%. Yield strength is measured in a range from 793 to 1645 MPa. It is anticipated that higher ultimate tensile strength and yield strength can be achieved in alloys herein by larger cold rolling reduction (>40%) that in our case is limited by laboratory mill capability. With more rolling force, it is anticipated that ultimate tensile strength could be increased to at least 2000 MPa and yield strength to at least 1800 MPa.
Tensile specimens were cut from cold rolled sheet samples by wire EDM and annealed at 850° C. for 10 min in a Lucifer 7HT-K12 box furnace. Samples were removed from the furnace at the end of the cycle and allowed to cool to room temperature in air. Results of tensile testing are shown in Table 24. As it can be seen, recrystallization during annealing of the alloys herein results in property combinations with ultimate tensile strength in the range from 939 to 1424 MPa and tensile elongation from 15.8 to 77.0%. Yield strength is measured in a range from 420 to 574 MPa.
This Case Example demonstrates that due to the unique mechanisms and structural pathway shown in
Slabs with thickness of 50 mm were laboratory cast from Alloy 1 and Alloy 2 according to the atomic ratios provided in Table 2 and hot rolled into sheets with final thickness of 2.31 mm for Alloy 1 sheet and 2.35 mm for Alloy 2 sheet. Casting and hot rolling procedures are described in Main Body section of current application. Resultant hot rolled sheet from each alloy was used for demonstration of cyclic structure/property reversibility through cold rolling/annealing cycles.
Hot rolled sheet from each alloy was subjected to three cycles of cold rolling and annealing. Sheet thicknesses before and after hot rolling and cold rolling reduction at each cycle are listed in Table 25. Annealing at 850° C. for 10 min was applied after each cold rolling. Tensile specimens were cut from the sheet in the initial hot rolled state and at each step of the cycling. Tensile properties were measured on an Instron 3369 mechanical testing frame using Instron's Bluehill control software. All tests were conducted at room temperature, with the bottom grip fixed and the top grip set to travel upwards at a rate of 0.012 mm/s Strain data was collected using Instron's Advanced Video Extensometer.
The results of tensile testing are plotted in
Tensile properties for each tested sample are listed in Table 26 and Table 27 for Alloy 1 and Alloy 2, respectively. As it can be seen, Alloy 1 has ultimate tensile strength from 1216 to 1238 MPa in hot rolled state with ductility from 50.0 to 52.7% and yield strength from 264 to 285 MPa. In cold rolled state, the ultimate tensile strength was measured in the range from 1482 to 1517 MPa at each cycle. Ductility was found consistently in the range from 28.5 to 32.8% with significantly higher yield strength of 718 to 830 MPa as compared to that in hot rolled condition. Annealing at each cycle resulted in restoration of the ductility to the range from 47.7 to 59.7% with ultimate tensile strength from 1216 to 1270 MPa. Yield strength after cold rolling and annealing is lower than that after cold rolling and was measured in the range from 431 to 515 MPa that is however higher than that in initial hot rolled condition.
Similar results with property reversibility between cold rolled and annealed material through cycling were observed for Alloy 2 (
This Case Example demonstrates that the High Strength Nanomodal Structure (Structure #3,
Slabs with thickness of 50 mm were laboratory cast from selected alloys listed in Table 28 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described in Main Body section of current application. Resultant sheet from each alloy with final thickness of ˜1.2 mm and Recrystallized Modal Structure (Structure #4,
Bend tests were performed using an Instron 5984 tensile test platform with an Instron W-6810 guided bend test fixture according to specifications outlined in the ISO 7438 International Standard Metallic materials—Bend test (International Organization for Standardization, 2005). Test specimens were cut by wire EDM to a dimension of 20 mm×55 mm×sheet thickness. No special edge preparation was done to the samples. Bend tests were performed using an Instron 5984 tensile test platform with an Instron W-6810 guided bend test fixture. Bend tests were performed according to specifications outlined in the ISO 7438 International Standard Metallic materials—Bend test (International Organization for Standardization, 2005).
The test was performed by placing the test specimen on the fixture supports and pushing with a former as shown in
The distance between supports, l, was fixed according to ISO 7438 during the test at:
Prior to bending, the specimens were lubricated on both sides with 3 in 1 oil to reduce friction with the test fixture. This test was performed with a 1 mm diameter former. The former was pushed downward in the middle of the supports to different angles up to 180° or until a crack appeared. The bending force was applied slowly to permit free plastic flow of the material. The displacement rate was calculated based on the span gap of each test in order to have a constant angular rate and applied accordingly.
Absence of cracks visible without the use of magnifying aids was considered evidence that the test piece withstood the bend test. If a crack was detected, the bend angle was measured manually with a digital protractor at the bottom of the bend. The test specimen was then removed from the fixture and examined for cracking on the outside of the bend radius. The onset of cracking could not be conclusively determined from the force-displacement curves and was instead easily determined by direct observation with illumination from a flashlight.
Results of the bending response of the alloys herein are listed in Table 28 including initial sheet thickness, former radius to sheet thickness ratio WO and maximum bend angle before cracking. All alloys listed in the Table 28 did not show cracks at 90° bend angle. The majority of the alloys herein have capability to be bent at 180° angle without cracking. Example of the samples from Alloy 1 after bend testing to 180° is shown in
In order to be made into complex parts for automobile and other uses, an AHSS needs to exhibit both bulk sheet formability and edge sheet formability. This Case Example demonstrates good bulk sheet formability of the alloys in Table 2 through bend testing.
Slabs with thickness of 50 mm were laboratory cast from selected alloys listed in Table 2according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Resultant sheet from each alloy with final thickness of 1.2 mm and Recrystallized Modal Structure (Structure #4,
Tensile specimens in the ASTM E8 geometry were prepared using both wire EDM cutting and punching. Tensile properties were measured on an Instron 5984 mechanical testing frame using Instron's Bluehill control software. All tests were conducted at room temperature, with the bottom grip fixed and the top grip set to travel upwards at a rate of 0.012 mm/s. Strain data was collected using Instron's Advanced Video Extensometer. Tensile data is shown in Table 29 and illustrated in
As can be seen from Table 30, EDM cutting is considered to be representative of the optimal mechanical properties of the identified alloys, without a sheared edge, and which were processed to the point of assuming Structure #4 (Recrystallized Modal Structure). Accordingly, samples having a sheared edge due to punching indicate a significant drop in ductility as reflected by tensile elongation measurements of the punched samples having the ASTM E8 geometry. For Alloy 1, tensile elongation is initially 47.2% and then drops to 8.1%, a drop itself of 82.8%%. The drop in ductility from the punched to the EDM cut (E2/E1) varies from 0.57 to 0.05.
The edge status after punching and EDM cutting was analyzed by SEM using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc. The typical appearance of the specimen edge after EDM cutting is shown for Alloy 1 in
This Case Example demonstrates that in a case of wire-EDM cutting tensile properties are measured at relative higher level as compared to that after punching. In contrast to EDM cutting, punching of the tensile specimens creates a significant edge damage which results in tensile property decrease. Relative excessive plastic deformation of the sheet alloys herein during punching leads to structural transformation to a Refined High Strength Nanomodal Structure (Structure #5,
Slabs with thickness of 50 mm were laboratory cast from selected alloys listed in Table 31 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Resultant sheet from each alloy with final thickness of 1.2 mm and Recrystallized Modal Structure (Structure #4,
Tensile specimens in the ASTM E8 geometry were prepared using both wire EDM cutting and punching. Part of punched tensile specimens was then put through a recovery anneal of 850° C. for 10 minutes, followed by an air cool, to confirm the ability to recover properties lost by punching and shearing damage. Tensile properties were measured on an Instron 5984 mechanical testing frame using Instron's Bluehill control software. All tests were conducted at room temperature, with the bottom grip fixed and the top grip set to travel upwards at a rate of 0.012 min/s. Strain data was collected using Instron's Advanced Video Extensometer. Tensile testing results are provided in Table 31 and illustrated in
For example, in the case of Alloy 1 indicated, when EDM cut into a tensile testing sample, a tensile elongation average value is about 47.2%. As noted above, when punched and therefore containing a sheared edge, the tensile testing of the sample with such edge indicated a significant drop in such elongation values, i.e. an average value of only about 8.1% due to Mechanism #4 and formation of Refined High Strength Nanomodal Structure (Structure 45
Punching of tensile specimens results in edge damage and lowering the tensile properties of the material. Plastic deformation of the sheet alloys herein during punching leads to structural transformation to a Refined High Strength Nanomodal Structure (Structure #5,
Slabs with thickness of 50 mm were laboratory cast from Alloy 1 and laboratory processed by hot rolling down to thickness of 2 mm and cold rolling with reduction of approximately 40%. Tensile specimens in the ASTM E8 geometry were prepared by wire EDM cut from cold rolled sheet. Part of tensile specimens was annealed for 10 minutes at different temperatures in a range from 450 to 850° C., followed by an air cool. Tensile properties were measured on an Instron 5984 mechanical testing frame using Instron's Bluehill control software. All tests were conducted at room temperature, with the bottom grip fixed and the top grip set to travel upwards at a rate of 0.012 mm/s. Strain data was collected using Instron's Advanced Video Extensometer. Tensile testing results are shown in
To show the microstructural recovery in accordance to the tensile property upon annealing, TEM studies were conducted on selected samples that were annealed at different temperatures. For comparison, cold rolled sheet was included as a baseline herein. Laboratory cast Alloy 1 slab of 50 mm thick was used, and the slab was hot rolled at 1250° C. by two-step of 80.8% and 78.3% to approx. 2 mm thick, then cold rolled by 37% to sheet of 1.2 mm thick. The cold rolled sheet was annealed at 450° C., 600° C., 650° C. and 700° C. respectively for 10 minutes.
One reason behind the difference in recovery and transition in deformation behavior is illustrated by the model TTT diagram in
In other words, in the broad context of the present invention, the effect of shearing and formation of a sheared edge, and its associated negative influence on mechanical properties, can be at least partially recovered at temperatures of 450° C. up to 650° C. as shown in
Accordingly, this Case Example demonstrates that upon deformation during cold rolling, concurrent processes occur involving dynamic strain hardening and phase transformation through unique Mechanisms #2 or #3 (
Slabs with thickness of 50 mm were laboratory cast from selected alloys listed in Table 33 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described in Main Body section of current application. Resultant sheet from each alloy with final thickness of 1.2 mm and Recrystallized Modal Structure (Structure #4,
Tensile specimens in the ASTM E8 geometry were prepared by punching. A part of punched tensile specimens from selected alloys was then put through a recovery anneal for 10 minutes at different temperatures in a range from 450 to 850° C., followed by an air cool. Tensile properties were measured on an Instron 5984 mechanical testing frame using Instron's Bluehill control software. All tests were conducted at room temperature, with the bottom grip fixed and the top grip set to travel upwards at a rate of 0.012 mm/s. Strain data was collected using Instron's Advanced Video Extensometer.
Tensile testing results are shown in Table 32 and in
Microstructural changes in Alloy 1 at the shear edge as a result of the punching and annealing at different temperatures were examined by SEM. Cross section samples were cut from ASTM E8 punched tensile specimens near the sheared edge in as-punched condition and after annealing at 650° C. and 700° C. as shown in
For SEM study, the cross section samples were ground on SiC abrasive papers with reduced grit size, and then polished progressively with diamond media paste down to 1 μm. The final polishing was done with 0.02 μm grit SiO2 solution. Microstructures were examined by SEM using an EVO-MA10 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
Punching of tensile specimens result in edge damage lowering the tensile properties of the material. Plastic deformation of the sheet alloys herein during punching leads to structural transformation to a Refined High Strength Nanomodal Structure (Structure #5,
Slabs with thickness of 50 mm were laboratory cast from selected alloys listed in Table 34 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Resultant sheet from each alloy with final thickness of 1.2 mm and Recrystallized Modal Structure (Structure #4,
Tensile specimens in the ASTM E8 geometry were prepared by punching at three different speeds of 28 mm/s, 114 mm/s, and 228 min/s. Wire EDM cut specimens from the same materials were used for the reference. A part of punched tensile specimens from selected alloys was then put through a recovery anneal for 10 minutes at 850° C., followed by an air cool. Tensile properties were measured on an Instron 5984 mechanical testing frame using Instron's Bluehill control software. All tests were conducted at room temperature, with the bottom grip fixed and the top grip set to travel upwards at a rate of 0.012 min/s. Strain data was collected using Instron's Advanced Video Extensometer. Tensile testing results are listed in Table 34 and tensile properties as a function of punching speed for selected alloys are illustrated in
This Case Example demonstrates that punching speed can have a significant effect on the resulting tensile properties in steel alloys herein. Localized heat generation at punching might be a factor in recovery of the structure near the edge leading to property improvement.
Slabs with thickness of 50 mm were laboratory cast from Alloy 1 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Resultant sheet with final thickness of 1.2 mm and Recrystallized Modal Structure (Structure #4,
Specimens for testing with a size of 89×89 mm were wire EDM cut from the sheet. The hole with 10 mm diameter was cut in the middle of specimens by utilizing two methods: punching and drilling with edge milling. The hole punching was done on an Instron Model 5985 Universal Testing System using a fixed speed of 0.25 mm/s with 16% clearance. Hole expansion ratio (HER) testing was performed on the SP-225 hydraulic press and consisted of slowly raising the conical punch that uniformly expanded the hole radially outward. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation. The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
Results of HER testing are shown in
Microhardness was measured for Alloy 1 at all relevant stages of the hole expansion process. Microhardness measurements were taken along cross sections of sheet samples in the annealed (before punching and HER testing), as-punched, and HER tested conditions. Microhardness was also measured in cold rolled sheet from Alloy 1 for reference. Measurement profiles started at an 80 micron distance from the edge of the sample, with an additional measurement taken every 120 microns until 10 such measurements were taken. After that point, further measurements were taken every 500 microns, until at least 5 mm of total sample length had been measured. A schematic illustration of microhardness measurement locations in HER tested samples is shown in
As shown in
To prepare the TEM specimens, the HER test samples were first sectioned by wire EDM, and a piece with a portion of hole edge was thinned by grinding with pads of reduced grit size. Further thinning to ˜60 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 near the edge of the hole and the final polishing was completed by electropolishing using a twin-jet polisher. The chemical solution used was a 30% Nitric acid mixed in Methanol base. In case of insufficient thin area for TEM observation, the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling usually is 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. Since the location for TEM study is at the center of the disc, the observed microstructure is approximately ˜1.5 mm from the edge of hole.
The initial microstructure of the Alloy 1 sheet before testing is shown on
To analyze in more detail the reason causing the poor HER performance in samples with punched holes, Focused Ion Beam (FIB) technique was utilized to make TEM specimens at the very edge of the punched hole. As shown in
Slabs with thickness of 50 mm were laboratory cast from selected alloys listed in Table 35 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Resultant sheet with final thickness of 1.2 mm and Recrystallized Modal Structure (Structure #4,
Test specimens of 89×89 mm were wire EDM cut from the sheet from larger sections. A 10 mm diameter hole was made in the center of specimens by punching on an Instron Model 5985 Universal Testing System using a fixed speed of 0.25 mm/s at 16% punch to die clearance. Half of the prepared specimens with punched holes were individually wrapped in stainless steel foil and annealed at 850° C. for 10 minutes before HER testing. Hole expansion ratio (HER) testing was performed on the SP-225 hydraulic press and consisted of slowly raising the conical punch that uniformly expanded the hole radially outward. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation.
The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
.The results of the hole expansion ratio measurements on the specimens with and without annealing after hole punching are shown in Table 35. As shown in
This Case Example demonstrates that edge formability demonstrated during HER testing can yield poor results due to edge damage during the punching operation as a result of the unique mechanisms in the alloys listed in Table 2. The fully post processed alloys exhibit very high tensile ductility as shown in Table 6 through Table 10 coupled with very high strain hardening and resistance to necking until near failure. Thus, the material resists catastrophic failure to a great extent but during punching, artificial catastrophic failure is forced to occur near the punched edge. Due to the unique reversibility of the identified mechanisms, this deleterious edge damage as a result of Nanophase Refinement & Strengthening (Mechanism #3,
In addition, it can be appreciated that the alloys herein that have undergone the processing pathways to provide such alloys in the form of Structure #4 (Recrystallized Modal Structure) will indicate, for a hole that is formed by shearing, and including a sheared edge, a first hole expansion ratio (HER1) and upon heating the alloy will have a second hole expansion ratio (HER2), wherein HER2>HER1.
More specifically, it can also be appreciated that the alloys herein that have undergone the processing pathways to provide such alloys with Structure #4 (Recrystallized Modal Structure) will indicate, for a hole that was placed in the alloy through methods (i.e. waterjet cutting, laser cutting, wire-edm, milling etc.) where the hole that is formed that does not rely primarily on shearing, compared to punching a hole, a first hole expansion ratio (HER1) where such value may itself fall in the range of 30 to 130%. However, when the same alloy includes a hole formed by shearing, a second hole expansion ratio is observed (HER2) wherein HER2=(0.01 to 0.30)(HER1). However, if the alloy is then subject to heat treatment herein, it is observed that HER2 is recovered to a HER3=(0.60 to 1.0) HER1.
Slabs with thickness of 50 mm were laboratory cast from Alloy 1 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Resultant sheet from Alloy 1 with final thickness of 1.2 mm and Recrystallized Modal Structure (Structure #4,
Tensile specimens of AS TM E8 geometry were created using two methods: Punching and wire EDM cutting. Punched tensile specimens were created using a commercial press. A subset of punched tensile specimens was heat treated at 850° C. for 10 minutes to create samples with a punched then annealed edge condition.
Tensile properties of ASTM E8 specimens were measured on an Instron 5984 mechanical testing frame using Instron's Bluehill 3 control software. All tests were conducted at room temperature, with the bottom grip fixed and the top grip set to travel upwards at a rate of 0.025 mm/s for the first 0.5% elongation, and at a rate of 0.125 mm/s after that point. Strain data was collected using Instron's Advanced Video Extensometer. Tensile properties of Alloy 1 with punched, EDM cut, and punched then annealed edge conditions are shown in Table 36. Tensile properties of Alloy 1 with different edge conditions are shown in
Specimens for hole expansion ratio testing with a size of 89×89 mm were wire EDM cut from the sheet. The holes with 10 mm diameter were prepared by two methods: punching and cutting by wire EDM. The punched holes with 10 mm diameter were created by punching at 0.25 mm/s on an Instron 5985 Universal Testing System with a 16% punch clearance and with using the flat punch profile geometry. A subset of punched samples for hole expansion testing were annealed with an 850° C. for 10 minutes heat treatment after punching.
Hole expansion ratio (HER) testing was performed on the SP-225 hydraulic press and consisted of slowly raising the conical punch that uniformly expanded the hole radially outward. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation.
The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
Hole expansion ratio testing results are shown in Table 37. An average hole expansion ratio value for each edge condition is also shown. The average hole expansion ratio for each edge condition is plotted in
This Case Example demonstrates that the edge condition of Alloy 1 has a distinct effect on the tensile properties and edge formability (i.e. HER response). Tensile samples tested with punched edge condition have diminished properties when compared to both wire EDM cut and punched after subsequent annealing. Samples having the punched edge condition have hole expansion ratios averaging 3.20%, whereas EDM cut and punched then annealed edge conditions have hole expansion ratios of 82.43% and 93.10%, respectively. Comparison of edge conditions also demonstrates that damage associated with edge creation (i.e. via punching) has a non-trivial effect on the edge formability of the alloys herein.
Slabs with thickness of 50 mm were laboratory cast from selected alloys listed in Table 38 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Resultant sheet from each alloy with final thickness of 1.2 mm and Recrystallized Modal Structure (Structure #4,
Specimens for testing with a size of 89×89 mm were wire EDM cut from the sheet. The holes with 10 mm diameter were punched at different speeds on two different machines but all of the specimens were punched with a 16% punch clearance and with the same punch profile geometry. The low speed punched holes (0.25 mm/s, 8 mm/s) were punched using an Instron 5985 Universal Testing System and the high speed punched holes (28 mm/s, 114 mm/s, 228 mm/s) were punched on a commercial punch press. All holes were punched using a flat punch geometry.
Hole expansion ratio (HER) testing was performed on the SP-225 hydraulic press and consisted of slowly raising the conical punch that uniformly expanded the hole radially outward. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation.
The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
Hole expansion ratio values for tests are shown in Table 37. An average hole expansion value is shown for each speed and alloy tested at 16% punch clearance. The average hole expansion ratio as a function of punch speed is shown in
This Case Example demonstrates a dependence of edge formability on punching speed as measured by the hole expansion ratio. As punch speed increases, the hole expansion ratio generally increases for the alloys tested. With increased punching speed, the nature of the edge is changed such that improved edge formability (i.e. HER response) is achieved. At punching speeds greater than those measured, edge formability is expected to continue improving towards even higher hole expansion ratio values.
Commercially produced and processed Dual Phase 980 steel was purchased and hole expansion ratio testing was performed. All specimens were tested in the as received (commercially processed) condition.
Specimens for testing with a size of 89×89 mm were wire EDM cut from the sheet. The holes with 10 mm diameter were punched at different speeds on two different machines but all of the specimens were punched with a 16% punch clearance and with the same punch profile geometry using a commercial punch press. The low speed punched holes (0.25 min/s) were punched using an Instron 5985 Universal Testing System and the high speed punched holes (28 mm/s, 114 mm/s, 228 min/s) were punched on a commercial punch press. All holes were punched using a flat punch geometry.
Hole expansion ratio (HER) testing was performed on the SP-225 hydraulic press and consisted of slowly raising the conical punch that uniformly expanded the hole radially outward. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation.
The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
Values for hole expansion tests are shown in Table 39. The average hole expansion value for each punching speed is also shown for commercial Dual Phase 980 material at 16% punch clearance. The average hole expansion value is plotted as a function of punching speed for commercial Dual Phase 980 steel in
This Case Example demonstrates that no edge performance effect based on punch speed is measureable in Dual Phase 980 steel. For all punch speeds measured on Dual Phase 980 steel the edge performance (i.e. HER response) is consistently within the 21%±3% range, indicating that edge performance in conventional AHSS is not improved by punch speed as expected since the unique structures and mechanisms present in this application as for example in
Slabs with thickness of 50 mm were laboratory cast from Alloys 1, 9, and 12 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Resultant sheet from each alloy with final thickness of 1.2 mm and Recrystallized Modal Structure (Structure #4,
Tested specimens of 89×89 mm were wire EDM cut from larger sections. A 10 mm diameter hole was punched in the center of the specimen at three different speeds, 28 mm/s, 114 min/s, and 228 mm/s at 16% punch clearance and with four punch profile geometries using a commercial punch press. These punch geometries used were flat, 6° tapered, 7° conical, and conical flat. Schematic drawings of the 6° tapered, 7° conical, and conical flat punch geometries are shown in
Hole expansion ratio (HER) testing was performed on the SP-225 hydraulic press and consisted of slowly raising the conical punch that uniformly expanded the hole radially outward. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation.
The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
Hole expansion ratio data is included respectively in Table 40, Table 41, and Table 42 for Alloy 1, Alloy 9, and Alloy 12 at four punch geometries and at two different punch speeds. The average hole expansion values for Alloy 1, Alloy 9, and Alloy 12 are shown in
This Case Example demonstrates that for all alloys tested, there is an effect of punch geometry on edge formability. For all alloys tested, the conical punch shapes resulted in the largest hole expansion ratios, thereby demonstrating that modifying the punch geometry from a flat punch to a conical punch shape reduces the damage within the material due to the punched edge and improves edge formability. The 7° conical punch geometry resulted in the greatest edge formability increase overall when compared to the flat punch geometry with the conical flat geometry producing slightly lower hole expansion ratios across the majority of alloys tested. For Alloy 1 the effect of punch geometry is diminished with increasing punching speed, with the three tested geometries resulting in nearly equal edge formability as measured by hole expansion ratio (
Hole expansion ratio testing was performed on commercial steel grades 780, 980 and 1180. All specimens were tested in the as received (commercially processed) sheet condition.
Specimens for testing with a size of 89×89 mm were wire EDM cut from the sheet of each grade. The holes with 10 mm diameter were punched at different speeds on two different machines with the same punch profile geometry using a commercial punch press. The low speed punched holes (0.25 mm/s) were punched using an Instron 5985 Universal Testing System at 12% clearance and the high speed punched holes (28 mm/s, 114 mm/s, 228 mm/s) were punched on a commercial punch press at 16% clearance. All holes were punched using a flat punch geometry.
Hole expansion ratio (HER) testing was performed on the SP-225 hydraulic press and consisted of slowly raising the conical punch that uniformly expanded the hole radially outward. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation.
The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
Results from hole expansion tests are shown in Table 43 through Table 45 and illustrated in
This Case Example demonstrates that no edge performance effect based on hole punch speed is measureable in tested commercial steel grades indicating that edge performance in conventional AHSS is not effected or improved by punch speed as expected since the unique structures and mechanisms present in this application as for example in
Existing steel materials have been shown to exhibit a strong correlation of the measured hole expansion ratio and the material's post uniform elongation. The post uniform elongation of a material is defined as a difference between the total elongation of a sample during tensile testing and the uniform elongation, typically at the ultimate tensile strength during tensile testing. Uniaxial tensile testing and hole expansion ratio testing were completed on Alloy land Alloy 9 on the sheet material at approximately 1.2 mm thickness for comparison to existing material correlations.
Slabs with thickness of 50 mm were laboratory cast of Alloy 1 and Alloy 9 according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling annealing at 850° C. for 10 min as described in the Main Body section of this application.
Tensile specimens in the ASTM E8 geometry were prepared by wire EDM. All samples were tested in accordance with the standard testing procedure described in the Main Body of this document. An average of the uniform elongation and total elongation for each alloy were used to calculate the post uniform elongation. The average uniform elongation, average total elongation, and calculated post uniform elongation for Alloy 1 and Alloy 9 are provided in Table 46.
Specimens for hole expansion ratio testing with a size of 89×89 mm were wire EDM cut from the sheet of Alloy 1 and Alloy 9. Holes of 10 mm diameter were punched at 0.25 mm/s on an Instron 5985 Universal Testing System at 12% clearance. All holes were punched using a flat punch geometry. These test parameters were selected as they are commonly used by industry and academic professionals for hole expansion ratio testing.
Hole expansion ratio (HER) testing was performed on the SP-225 hydraulic press and consisted of slowly raising the conical punch that uniformly expanded the hole radially outward. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation.
The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
The measured hole expansion ratio values for Alloy 1 and Alloy 9 are provided in Table 46.
Commercial reference data is shown for comparison in Table 47 from [Paul S. K., J Mater Eng Perform 2014; 23:3610.]. For commercial data, S. K. Paul's prediction states that the hole expansion ratio of a material is proportional to 7.5 times the post uniform elongation (See Equation 1).
HER=7.5(εpul) Equation 1
The Alloy 1 and Alloy 9 post uniform elongation and hole expansion ratio are plotted in
This Case Example demonstrates that for the steel alloys herein, the correlation between post uniform elongation and the hole expansion ratio does not follow that for commercial steel grades. The measured hole expansion ratio for Alloy 1 and Alloy 9 is much smaller than the predicted values based on correlation for existing commercial steel grades indicating an effect of the unique structures and mechanisms are present in the steel alloys herein as for example shown in
Slabs with thickness of 50 mm were laboratory cast from three selected alloys according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Sheet from each alloy possessing the Recrystallized Modal Structure with final thickness of 1.2 mm were used to demonstrate the effect of hole expansion speed on HER performance.
Specimens for testing with a size of 89×89 mm were cut via wire EDM from the sheet. Holes of 10 mm diameter were punched at a constant speed of 228 mm/s on a commercial punch press. All holes were punched with a flat punch geometry, and with approximately 16% punch to die clearance.
Hole expansion ratio (HER) testing was performed on an Interlaken Technologies SP-225 hydraulic press and consisted of raising the conical punch that uniformly expanded the hole radially outward. Four hole expansion speeds, synonymous with the conical ram travel speed, were used; 5, 25, 50, and 100 mm/min. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation.
The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
Hole expansion ratio values for the tests are shown in Table 48. The average hole expansion ratio value is shown for each speed and alloy tested showing an increase in HER values with increasing hole expansion speed in all three alloys. The effect of hole expansion speed is also demonstrated in
This Case Example demonstrates that formability of the edge, i.e. its ability to be deformed with relatively reduced cracking, as measured by HER testing, can be affected by the speed of deformation of the hole edge (i.e. hole expansion speed). The alloys tested in this Case Example demonstrated a positive correlation between hole expansion ratio and the hole expansion speed, with increasing hole expansion speed resulting in relatively higher measured hole expansion ratios.
Accordingly, in the broad context of the present disclosure, it has been established that once an edge is formed, of any geometry by any edge formation method which causes deformation of the metal alloy when forming the edge (e.g. by punching, shearing, piercing, perforating, cutting, cropping, stamping,), by increasing the speed at which that edge once formed is then expanded, one observes that the edge itself is then capable of more expansion with a relatively reduced tendency to crack. The edge herein can therefore include an edge that defines an internal hole in a metal sheet of the alloys described herein, or an external edge on such metal sheet. In addition, the edge herein may be formed in a progressive die stamping operation which is reference to metal working operation that typically includes punching, shearing, coining and bending. The edge herein may be present in a vehicle, or more specifically, part of a vehicular frame, vehicular chassis, or vehicle panel.
Reference to edge expansion herein is understood as increasing the length of such edge with a corresponding change in the thickness of the edge. That is confirmed by the above data in Table 48, which shows that with respect to an edge that is present in a hole, when such edge in the hole is expanded at a speed of greater than or equal to 5 min/min, one observes an increase in the hole expansion ratio (i.e. the edge in the hole is capable of expansion to higher percentages over the original diameter) and the edge getting thinner as shown for example in the cross sections of the expanded edges in
Sheet from Alloy 9 was produced according to the atomic ratios provided in Table 2. Slabs produced by continuous casting were hot rolled into hot band which was subsequently processed into sheet with thickness of approximately 1.4 mm by cold rolling and annealing cycles. The microstructure of the produced sheet using both SEM and etched optical microscopy is demonstrated in
In
The sheet with Recrystallized Modal Structure was used for HER testing. Specimens for testing with a size of 89×89 mm were cut via wire EDM from the sheet. Holes of 10 mm diameter were punched at two different speeds of 5 mm/s using an Instron mechanical test frame and at 228 mm/s using a commercial punch press with a flat punch geometry and with punch to die clearances of approximately 12.5% and 16%, respectively.
Hole expansion ratio (HER) testing was performed on an Interlaken Technologies SP-225 hydraulic press and consisted of raising the conical punch that uniformly expanded the hole radially outward. Two hole expansion speeds of 3 mm/min and 50 mm/min, synonymous with the conical ram travel speed, were used. A digital image camera system was focused on the conical punch and the edge of the hole was monitored for evidence of crack formation and propagation.
The initial diameter of the hole was measured twice with calipers, measurements were taken at 90° increments and averaged to get the initial hole diameter. The conical punch was raised continuously until a crack was observed propagating through the specimen thickness. At that point the test was stopped and the hole expansion ratio was calculated as a percentage of the initial hole diameter measured before the start of the test. After expansion four diameter measurements were taken using calipers every 45° and averaged to account for any asymmetry of the hole due to cracking.
Hole expansion ratio values for tests are listed in Table 49. HER values vary from 2.4 to 18.5% in the samples with holes punched at 5 mm/s. In the case of 228 mm/s hole punching speed, HER values are significantly higher in a range from 33.8 to 75.0%. The effect of expansion speed is illustrated in
The magnetic phases volume percent (Fe %) was measured in the HER tested samples with different hole punching speed and hole expansion speed using a Fischer Feritscope FMP30. The results are listed in Table 50.
This Case Example illustrates that the relative resistance to cracking of an edge as confirmed by HER testing can be increased by, in the exemplary case of forming an edge within a hole, by either increasing hole punching speeds, hole expansion speeds or both. The sheet from Alloy 9, tested in this Case Example, demonstrated an increase in hole expansion ratio with increasing hole punching speed (i.e. 5 to 228 mm/s) and/or the hole expansion speed (i.e. 3 to 50 mm/min). Accordingly, preferably herein for the subject alloys, one forms an edge in the alloy and expands the edge at a speed of greater than or equal to 5 mm/min. The magnetic phases volume percent (Fe %) in tested samples increases with increasing hole punching speed and/or the hole expansion speed over the ranges studied. With this relatively greater amount of deformation available in and adjacent to the hole edge during the now disclosed increased hole punching speed or hole expansion speed, the higher local formability and resistance to cracking of the edge is achieved in the material as measured by the HER.
Slabs with thickness of 50 mm were laboratory cast from three selected alloys according to the atomic ratios provided in Table 2 and laboratory processed by hot rolling, cold rolling and annealing at 850° C. for 10 min as described herein. Sheet from each alloy possessing the Recrystallized Modal Structure with final thickness of 1.2 mm were used to demonstrate an effect of hole expansion speed on HER performance.
Specimens for testing with a size of 89×89 mm were cut via wire EDM from the sheet. A 10 mm diameter hole was prepared by various methods including punching, EDM cutting, milling, and laser cutting. Hole punching was done at a low quasistatic punching speed of 0.25 mm/s at 16% punch to die clearance using a Komatsu OBS80-3 press. EDM cut holes were first rough cut then the final cut was made at parameters to yield a visually smooth surface. During hole milling, holes were pilot drilled, reamed to size, and then deburred. Laser cut samples were cut on a 4 kW fiber optic Mazak Optiplex 4020 Fiber II machine.
Hole expansion ratio (HER) testing was performed on an Interlaken Technologies SP-225 hydraulic press and consisted of raising the conical punch that uniformly expanded the hole radially outward. In
In
In
This application is a continuation of U.S. patent application Ser. No. 16/522,708, filed Jul. 26, 2019, which is a continuation of U.S. patent application Ser. No. 15/438,313 filed Feb. 21, 2017, now U.S. Pat. No. 10,465,260, which is a continuation-in-part of U.S. patent application Ser. No. 15/094,554 filed Apr. 8, 2016, now U.S. Pat. No. 10,480,042, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/146,048 filed on Apr. 10, 2015 and U.S. Provisional Patent Application Ser. No. 62/257,070 filed on Nov. 18, 2015, all of which are fully incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62146048 | Apr 2015 | US | |
| 62257070 | Nov 2015 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16522708 | Jul 2019 | US |
| Child | 17228537 | US | |
| Parent | 15438313 | Feb 2017 | US |
| Child | 16522708 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15094554 | Apr 2016 | US |
| Child | 15438313 | US |
