Patent Application
20190003003
This disclosure is related to retention of mechanical properties in high strength steel at reduced thicknesses and which mechanical property performance is also retained at relatively high strain rates. These new steels can offer advantages for a myriad of applications where reduced sheet thickness is desirable. In addition, the alloys herein are those that retain useful mechanical properties after introduction of a geometric discontinuity and an accompanying stress concentration.
Steel is the engineering material of choice where cost, strength, and ductility are major factors. Accordingly, steel continues to be used in a myriad of applications in our daily lives, including in the construction of buildings, appliances, and automobiles. A large variety of steel alloys exist to achieve this range of needs, with targeted property ranges used for these wide ranging applications. Designations are provided for ranges of steel, which fit three distinct 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). Advanced High Strength Steels (AHSS) are of primary interest for advanced engineering applications, and are classified by 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 tensile strengths. For example, tensile elongation of LSS, HSS and AHSS ranges from 25% to 55%, 10% to 45%, and 4% to 30%, respectively.
An area where steel provides particular engineering advantages is in automobiles, with many different types of steels utilized throughout the car in various locations. Current consumer desires and governmental regulations are pushing automobile manufacturers to design vehicles that attain ever greater fuel efficiency. Automobile designers have identified weight reduction, particularly in the body-in-white structure, to have the greatest potential impact on improving fuel efficiency. The process of reducing automobile weight, known as lightweighting, can be accomplished through reducing the thickness of the body-in-white structure and increasing the geometric complexity of the various parts using high strength, high formability materials. Accordingly, increasingly high strength steels are desired throughout the automobile assembly in order to enable the thickness reduction and weight savings.
Safety must be kept constant or improved during the lightweighting process as well. Automobile highway speed limits are regularly increasing, and consumers expect safety performance to be a major part of automobile design. The body-in-white structure of an automobile is designed to provide a rigid structure that will protect the passenger while traveling at speed and in the case of a collision. During an automobile collision, dynamic loading, rapid deformation, and energy dissipation occurs throughout the automobile and body-in-white structure in particular. The time frame over which this occurs can be 100 ms. High strain rates are observed throughout the body-in-white structure during this time, and materials need to be able to withstand complex loads across a range of strain rates. For instance, a low speed collision that occurs in a parking lot would result in a lower strain rate for body-in-white than would a collision at highway speeds. The mechanical properties of materials for the body-in-white structure are measured by many means, including uniaxial tensile testing, across this range of strain rates such that their response during a collision can be predicted and design considerations taken into account. High strain rates can result in a change in mechanical properties, limiting the maximum lightweighting that automobile designers are able to achieve by requiring additional thickness to maintain safety under high strain rate conditions.
As advances in engineering and technology occur, there is an increasing drive to the small scale. Consumers, and by extension engineers/designers, are regularly searching for products that are size efficient. Consumers seek out products that accomplish the needed task while occupying the smallest volume possible. A good example of this phenomenon can be found in the electronics industry, where cell phones, tablets, and other devices are regularly reduced in size with consecutive design iterations. With the drive of products to smaller and smaller sizes, the demands on engineering materials that the products are made from increase dramatically. As the overall size of a part decreases, defects that are inherent in everyday manufacturing processes can result in significant reductions in material properties. High strength materials are particularly impacted by the reduction of part size to the small or very small due to the complex and often specialized processing required to achieve those properties.
Martensitic steels, for example, provide excellent strength yet require a quench as a final processing step to create the necessary microstructure. Quenching is difficult to control at a small scale and may potentially cause unacceptable distortion in small parts. Final processing may not be performed on the final part geometry but rather on sheet or foils in some applications. For thermally sensitive materials such as martensitic steels, thermal exposure during cutting to produce the final part may detrimentally alter the microstructure and compromise properties. Geometry effects also play a greater role in mechanical properties of ductile materials at the small scale, with the effects of stress concentrators, grain size, and thickness adversely changing the material's mechanical response to stress. Due to these facts, expensive engineering materials are often required for uses on small scale that are either thermally insensitive or have simple processing such as low alloyed or pure materials. Engineers would prefer to not use exotic materials for these applications; however everyday engineering materials are often unavailable for use at reduced thicknesses resulting in the slow adoption of smaller devices due to prohibitive cost and processing requirements.
In one embodiment, the present invention is directed at a method to retain mechanical properties in a metallic sheet alloy at reduced thickness comprising supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu, or C, melting said alloy, cooling at a rate of <250 K/s, and solidifying to a thickness of 25.0 mm up to 500 mm. This is followed by processing the alloy into sheet form with thickness T1 with the sheet having a total elongation of X1 (%), an ultimate tensile strength of Y1 (MPa), and a yield strength of Z1 (MPa). This is then followed by further processing the alloy into a second sheet with reduction in thickness T2<T1 with the second sheet having a total elongation of X2=X1±10%, an ultimate tensile strength of Y2=Y1±50 MPa, and a yield strength of Z2=Z1±100 MPa.
In another embodiment the present invention relates to a method to retain mechanical properties in a metallic sheet alloy at relatively high strain rates comprising supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu, or C and melting said alloy and cooling at a rate of <250 K/s and solidifying to a thickness of 25.0 mm up to 500 mm. This is then followed by processing the alloy into sheet form with thickness from 1.2 mm to 10.0 mm with the sheet having a total elongation of X1 (%), an ultimate tensile strength of Y1 (MPa), and a yield strength of Z1 (MPa) when tested at a strain rate S1. This is then followed by deforming the sheet from the alloy at a strain rate S2>S1 with the sheet having a total elongation of X3=X1±7%, ultimate tensile strength Y3=Y1±200 MPa, and yield strength Z3=Z1±50 MPa.
In yet another embodiment the present invention is directed at A method to retain mechanical properties in a metallic sheet alloy comprising supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu, or C and melting said alloy and cooling at a rate of <250 K/s and solidifying to a thickness of 25.0 mm up to 500 mm. This is then followed by processing the alloy into sheet form with thickness from 1.2 mm to 10.0 mm with the sheet having a total elongation of X1 (%), an ultimate tensile strength of Y1 (MPa), and a yield strength of Z1 (MPa). Then, one may introduce stress concentration sites and then deform the sheet from the alloy with the sheet having a total elongation of X4≤0.2X1 (%), an ultimate tensile strength Y4≤0.5Y1 (MPa), and a yield strength Z4≥0.6Z1 (MPa).
The detailed description below may be better understood with reference to the accompanying FIG.s which are provided for illustrative purposes and are not to be considered as limiting any aspect of this invention.
The retention of mechanical properties in the alloys herein at reduced thickness and relatively high strain rates is illustrated in
The steps to produce this sheet at thickness T1 from the cast product can vary depending on specific manufacturing routes and specific targeted goals. As an example, consider thick slab casting as one process route to get to sheet of this targeted thickness. The alloy would be cast going through a water cooled mold typically in a thickness range of 150 to 300 mm in thickness. The cast ingot after cooling would then be preferably prepared for hot rolling which may involve some surface treatment to remove surface defects including oxides. The ingot would then go through a roughing mill hot roller which may involve several passes resulting in a transfer bar slab typically from 15 to 100 mm in thickness. This transfer bar would then go through successive/tandem hot rolling finishing stands to produce hot band coils which have a thickness T1 in the above referenced range from 1.2 mm to 10.0 mm.
Another example would be to preferably process the cast material through a thin slab casting process. In this case, after casting typically forms 35 to 150 mm in thickness by going through a water cooled mold, the newly formed slab goes directly to hot rolling without cooling down with auxiliary tunnel furnace or induction heating applied to bring the slab directly up to targeted temperature. The slab is then hot rolled directly in multi-stand finishing mills which are preferably from 1 to 7 in number. After hot rolling, the strip is rolled into hot band coils with thickness T1 in the above referenced range of 1.2 mm to 10.0 mm in thickness. Note that bloom casting would be similar to the examples above but higher thickness might be cast typically from 200 to 500 mm thick and initial breaker steps would be needed to reduce initial cast thickness to allow it to go through a hot rolling roughing mill. Strip casting would be similar but lower thickness might be cast of T1 having a value of 1.2 mm to 10.0 mm in thickness with preferably only one hot rolling stand directly after casting.
Accordingly, the specific process in going from the slab material in Step 1 to a preferred thickness T1 of 1.2 mm to 10 mm and then in Step 2 to a preferred thickness in the range of 0.2 mm to less than 1.2 mm may include hot rolling, cold rolling, and/or cold rolling followed by annealing. Accordingly, in Step 2, the alloy thickness may preferably be 0.2 mm, 0.3 mm, 0.4 mm. 0.5 mm. 0.6 mm. 0.7 mm. 0.8 mm. 0.9 mm, 1.0 mm 1.1 mm up to by not including 1.2 mm. Hot rolling is generally used to provide a preferred thickness from 1.2 mm to 10.0 mm and is typically done in roughing mills, finishing mills, and/or Steckel mills. Cold rolling is preferred in Steps 1 and/or Step 2 and is generally done using tandem mills, Z-mills, and/or reversing mills. The cold rolled material depending on property targets may be annealed to restore the ductility lost from the cold rolling process either partially or with restoration of ductility. Typically as cold rolling proceeds and higher amounts of gauge reduction occurs, ductility is reduced and cold rolling will continue until or just before cracking is observed. Restoration of the tensile ductility of the cold rolled sheet generally occurs with heat treatments at 700° C. and above. Once the sheet is formed with thickness T1 specified in Step 2, the sheet will then exhibit a total elongation of X1 (%), an ultimate tensile strength of Y1 (MPa), and a yield strength of Z1 (MPa). Preferred properties for alloys herein in Step 2 would be tensile elongation from 12 to 80%, ultimate tensile strength values from 700 to 2100 MPa, and yield strength is in a range from 250 to 1500 MPa.
In Step 3, the alloy is preferably cold rolled and annealed in similar manner as in Step 2 to thickness T2<T1. In Step 3, comparing said alloy in Step 1 and after Step 2, the total elongation is maintained at the level where the total elongation X2=X1±10%, Y2=Y1±50 MPa, and Z2=Z1±100 MPa. The thickness of the alloy in Step 3 is identified as T2 and is less than the thickness T1 in Step 2. The preferred properties of the alloy in Step 3 are as follows: X2=2 to 90%; Y2=650 MPa to 2150 MPa and Z2=150 MPa to 1600 MPa.
Alloys herein are also shown to avoid brittle fracture when stress concentration sites are introduced such as notches at the sheet edge. A stress concentration site herein is a location on the alloy sheet where stress can be concentrated, including but not limited to geometric discontinuities, such as a notch, hole, cut in the surface, crack, chipped portion, dent, etc.
The chemical composition of the alloys herein is shown in Table 1 which provides the preferred atomic ratios utilized.
As can be seen from Table 1, the alloys herein comprise, consist essentially of, or consist of iron based metal alloys, having greater than 70 at. % Fe, and at least four or more elements selected from the following six (6) elements: Si, Mn, Cr, Ni, Cu, and C. The level of impurities of other elements are in the range of 0 to 5000 ppm. Accordingly, if there is 5000 ppm of an element other than the selected elements identified, the level of such selected elements may then in combination be present at a lower level to account for the 5000 ppm impurity, such that the total of all elements present (selected elements and impurities) is 100 atomic percent.
With regards to the above, and as can be further seen from Table 1, preferably, when Fe is present at a level of greater than 70 at. %, and one then selects the four or more elements from the indicated six (6) elements, or selects five or more elements, or selects all six elements to provide a formulation of elements that totals 100 atomic percent. The preferred levels of the elements, if selected, may fall in the following ranges: Si (1.14 to 6.13), Mn (3.19 to 15.17), Cr (0.78 to 8.64); Ni (0.9 to 11.44), Cu (0.37 to 1.87), and C (0.67 to 3.68). Accordingly, it can be appreciated that if four (4) elements are selected, two of the six elements are not selected and may be excluded. If five (5) elements are selected, one of the elements of the six can be excluded. Moreover, a particularly preferred level of Fe is in the range of 73.95 to 84.69 at. %. Again, the level of impurities of other elements are preferably controlled in the range of 0 to 5000 ppm (0 to 0.5 wt %).
Alloys were weighed out into 3,000 to 3,400 gram charges according to the atomic ratios in Table 1 using commercially available ferroadditive powders and a base steel feedstock with known chemistry. As alluded to above, impurities can be present at various levels depending on the feedstock used. Impurity elements would commonly include the following elements; Al, Co, N, P, Ti, Mo, W, Ga, Ge, Sb, Nb, Zr, O, Sn, Ca, and S which if present would be in the range from 0 to 5000 ppm (parts per million) (0 to 0.5 wt %) at the expense of the desired elements noted above. Preferably, the level of impurities is controlled to fall in the range of 0 to 3000 ppm (0.3 wt %).
Charges were loaded into a zirconia coated silica crucible which was placed into an Indutherm VTC800V vacuum tilt casting machine. The machine then evacuated the casting and melting chambers and flushed with argon to atmospheric pressure twice prior to casting to prevent oxidation of the melt. The melt was heated with a 14 kHz RF induction coil until fully molten, approximately from 5 to 7 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 chamber and tilted the crucible and poured the melt into 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.
Laboratory casting corresponds to Step 1 in
A sample of between 50 and 150 mg from each alloy herein was taken in the as-cast condition. This sample was heated to an initial ramp temperature between 900° C. and 1300° C. depending on alloy chemistry, at a rate of 40° C./min. Temperature was then increased at 10° C./min to a max temperature between 1425° C. and 1515° C. depending on alloy chemistry. Once this maximum temperature was achieved, the sample was cooled at a rate of 10° C./min back to the initial ramp temperature before being reheated at 10° C./min to the maximum temperature. Differential Scanning calorimetry (DSC) measurements were taken using a Netzsch Pegasus 404 DSC through all four stages of the experiment, and this data was used to determine the solidus and liquidus temperatures of each alloy, which are in a range from 1102 to 1505° C. (Table 2). Depending on alloys chemistry, liquidus-solidus gap varies from 31 to 138° C. Thermal analysis provides information on maximum temperature for the following hot rolling processes that varies depending on alloy chemistry.
The alloys herein were preferably processed into a laboratory hot band by hot rolling of laboratory slabs at high temperatures. Laboratory alloy processing is developed to simulate the hot band production from slabs produced by continuous casting. Industrial hot rolling is performed by heating a slab in a tunnel furnace to a target temperature, then passing it through either a reversing mill or a multi-stand mill or a combination of both to reach the target gauge. During rolling on either mill type, the temperature of the slab is steadily decreasing due to heat loss to the air and to the work rolls so the final hot band is formed at a reduced temperature. This is simulated in the laboratory by heating in a tunnel furnace to between 1100° C. and 1250° C., then hot rolling. The laboratory mill is slower than industrial mills causing greater loss of heat during each hot rolling pass so the slab is reheated for 4 minutes between passes to reduce the drop in temperature, the final temperature at target gauge when exiting the laboratory mill commonly is in the range from 800° C. to 1000° C., depending on furnace temperature and final thickness.
Prior to hot rolling, laboratory slabs were preheated in a Lucifer EHS3GT-B18 furnace. The furnace set point varies between 1100° C. to 1250° C., depending on alloy melting point and point in the hot rolling process, with the initial temperatures set higher to facilitate higher reductions, and later temperatures set lower to minimize surface oxidation on the hot band. The slabs were allowed to soak for 40 minutes prior to hot rolling to ensure they reach the target temperature and then 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 10 passes though the mill before being allowed to air cool. Final thickness ranges after hot rolling are preferably from 1.8 mm to 4.0 mm with variable reduction per pass ranging from 20% to 50%.
Tensile specimens were cut from laboratory hot band using wire EDM. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. Samples were tested under displacement control at a constant displacement rate of 0.036 mm/s, which resulted in sample strain rates, calculated from video strain measurements, ranging from 4.4×10−4 s−1 to 6.8×10−3 s−1, depending on several factors including, but not always limited to mechanical compliance, sample slippage, and settling of the wedge action grips used.
Tensile properties of the alloys in the hot rolled condition with a thickness from 1.8 to 2.3 mm are listed in Table 3 including magnetic phases volume percent (Fe %) that was measured by Feritscope. The ultimate tensile strength values may vary from 913 to 2011 MPa with tensile elongation from 13.0 to 69.5%. The yield strength is in a range from 250 to 1313 MPa. Mechanical properties of the hot band from steel alloys herein depend on alloy chemistry, processing conditions, and material mechanistic response to the processing conditions. The relative magnetic phases volume percent was measured by Feritscope with the magnetic phases volume percent of 0.1 to 64.9 Fe % in a hot band depending on alloy chemistry. Note that the Table 3 properties correspond to Step 2 of
The hot band from alloys herein listed in Table 1 was cold rolled to final target gauge thickness of 1.2 mm through multiple cold rolling passes. Cold rolling is defined as rolling at ambient temperature. Hot band material was media blasted prior to cold rolling to remove surface oxides which could become embedded during the rolling process. The resultant cleaned sheet material was rolled using a Fenn Model 061 2 high rolling mill. Sheet was fed through the rolls, and the roll gap is reduced for each subsequent pass until the desired thickness is achieved or the material hardens to the point where additional rolling does not achieve significant reduction in thickness. Annealing was applied before next rolling to recover ductility. Multiple cycles of cold rolling and annealing might be applied. Once the final gauge thickness was reached, samples were cut from each cold rolled sheet by wire EDM. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control. Samples were tested under displacement control at a constant displacement rate of 0.036 mm/s, which resulted in sample strain rates, calculated from video strain measurements, ranging from 4.4×10−4 s−1 to 6.8×10−3 s−1, depending on several factors including, but not always limited to mechanical compliance, sample slippage, and settling of the wedge action grips used.
Tensile properties of 1.2 mm thick sheet from alloys herein after cold rolling are listed in Table 4. The ultimate tensile strength values after cold rolling is in a range from 1360 to 2222 MPa; yield strength varies from 1006 to 2073 MPa and tensile elongation is recorded in the range from 4.2 to 37.2%. The magnetic phases volume percent was measured by Feritscope in a range from 1.6 to 84.9 Fe % in a cold rolled sheet depending on alloy chemistry.
The samples were annealed under conditions intended to simulate the thermal exposure expected during an industrial continuous annealing process representing final treatment of sheet material in Step 2 in
Tensile properties of 1.2 mm sheet from alloys herein after annealing are listed in Table 5. The ultimate tensile strength values of the annealed sheet from alloys herein is in a range from 725 to 2072 MPa; yield strength varies from 267 to 1428 MPa and tensile elongation is recorded in the range from 12.8 to 76.9%. The relative magnetic phases volume percent was measured by Feritscope with the magnetic phases volume percent of 0.2 to 68.2 Fe % depending on alloy chemistry.
Properties of cold rolled and annealed sheet from Alloys herein corresponds to Step 2 in
This Case Example demonstrates properties of the sheet material from alloys herein with thickness of 1.2 to 1.4 mm and tested at strain rates from 4.4×10−4 s−1 to 6.8×10−3 s−1.
The hot band from Alloy 2 was cold rolled into sheets with different thicknesses through multiple cold rolling passes. Once the targeted gauge thickness was reached, samples were cut from each cold rolled sheet by wire EDM. The samples were annealed under conditions intended to simulate the thermal exposure expected during an industrial continuous annealing process. Samples were wrapped in stainless steel foil to prevent oxidation and loaded into a preheated furnace at 850° C. Samples were left in the furnace for 10 minutes while the furnace purged with argon before being removed and allowed to air cool. The only exception was the final anneal for the 4.8 mm material. This anneal was an 850° C. 20 min air cooled anneal, as opposed to the 10 minute anneal used for every other thickness. The purpose of this change was to allow more time for the material to heat up as it was a much thicker sample. Tensile properties were measured on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control. All samples were tested at displacement rate of 0.125 mm/s, which resulted in sample strain rates, calculated from video strain measurements, ranging from 9.1×10−4 s−1 to 1.9×10−3 s−1 depending on several factors including, but not always limited to mechanical compliance, sample slippage, and settling of the wedge action grips used.
The results of tensile testing of the sheet from Alloy 2 processed to different thicknesses are listed in Table 6. In samples with thickness less than 1.2 mm representing Step 3 in
This Case Example demonstrates that high ductility maintained in the sheet with thickness in a wide range from 4.8 mm down to as small as 0.2 mm. Reduction in sheet thickness below 1.2 mm results in an average total elongation that is no less than that in the sheet with 1.2 mm thickness and above minus 7.8%. An average ultimate tensile strength is 25 MPa less than that in the corresponding sheet with 1.2 mm thickness and above and average yield strength is 67 MPa less.
The hot band from Alloy 1, Alloy 27, and Alloy 37 was cold rolled in to sheets with different thicknesses less than 1.2 mm through multiple cold rolling passes. Once the targeted gauge thickness was reached, samples were cut from each cold rolled sheet by wire EDM. The samples were annealed under conditions intended to simulate the thermal exposure expected during an industrial continuous annealing process representing final treatment at sheet processing in Step 2 in
The results of tensile testing of the sheet from the alloys processed to different thicknesses are listed in Table 7 representing Step 3 in
This Case Example demonstrates that tensile ductility of alloys herein is maintained even at sheet thickness as small as 0.2 mm demonstrating an average total elongation no less than that in the corresponding sheet with 1.2 mm thickness and above minus 7.3%. An average ultimate tensile strength is a range of ±35 MPa of that in the corresponding sheet with 1.2 mm thickness and above with the yield strength in a range of ±98 MPa.
The hot band from Alloy 1, Alloy 2, Alloy 27, and Alloy 37 was cold rolled in to sheets with different thicknesses less than 1.2 mm through multiple cold rolling passes. Once the targeted gauge thickness was reached, samples were cut from each cold rolled sheet by wire EDM. The samples were annealed under conditions intended to simulate the thermal exposure expected during an industrial continuous annealing process. Samples were wrapped in stainless steel foil to prevent oxidation and loaded into a preheated furnace at 850° C. The microstructures of the cold rolled and annealed state were studied by SEM to show the structural change during processing. To prepare SEM samples, pieces were cut by EDM from the sheet and mounted in epoxy, and the sheet cross-sections were polished progressively with 9 μm, 6 μm and 1 μm diamond suspension solution, and finally with 0.02 μm silica. The SEM study was conducted using an EVO-60 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
The structure in the sheet samples from Alloy 27 is similar to Alloy 1 and Alloy 2 and is shown in
Alloy 37 is a different type of the alloy in which the annealing does not lead to the typical recrystallized structure formation.
This Case Example demonstrates that microstructure is maintained in alloys herein after annealing of cold rolled sheet independently from the final sheet thickness.
Slabs of Alloy 2 were cast according to the atomic compositions provided in Table 1. Following casting, the slabs were hot rolled through successively smaller roll gaps to produce hot band coils in the range of 2 to 5 mm thick, which were subsequently subjected to cold rolling and annealing cycles until the targeted thickness of approximately 1.4 mm was achieved representing sheet material in Step 2 in
The tensile properties of the material were characterized as a function of strain rate. Tensile samples were tested at 0.0007 s−1, 0.7 s−1, 10 s−1, 100 s−1, 500 s−1 and 1200 s−1 nominal strain rates in the ASTM D638 Type V tensile geometry shown in
Strain in the tensile samples was measured by a mechanical extensometer at 0.0007 s−1 and 0.7 s−1 strain rates. Digital Image Correlation (DIC) was used to measure strain for samples tested at 10 s−1, 100 s−1, and 500 s−1. Five tensile samples were tested at all strain rates. In the case of one sample at 0.0007 s−1 strain rate, a malfunction occurred that resulted in the loss of the sample. Two samples tested at 1200 s−1 did not fail during testing.
Measured strain at failure is provided in Table 8. The measured strain is plotted as function of strain rate in
Tensile properties in Tables 8 through 10 represents sheet material in Step 3 in
This Case Example demonstrates that tensile ductility of alloys herein is retained across a relatively large range of strain rates of 0.007 to 1200 s−1. A measured average ultimate tensile strength is 62 MPa lower at higher strain rates and average yield strength is 59 MPa lower.
The microstructures of the samples from sheet from Alloy 2 tested at five different strain rates ranging from 0.0007 s−1 to 1200 s−1 (see Case Example #5) were studied by TEM. For TEM study, pieces are cut from the gauge section of deformed samples by diamond saw. Grinding and polishing are then undertaken to make thin foils from the cut pieces. The polishing was conducted progressively with 9 μm, 6 μm and 1 μm diamond suspension solution, and finally with 0.02 μm silica. Foils with thickness of 70 to 80 μm were obtained after the polishing. 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.
This Case Example demonstrates the alteration of deformation mechanisms during deformation of the alloys herein with higher occurrence of twinning with increasing strain rate. Deformation by twinning at high strain rates suppresses the phase transformation (i.e. means that the total amount of ferrite produced is reduced) allowing to the retention of relatively high tensile ductility of the sheet material in a wide range of strain rates.
Slabs of Alloy 2 were cast according to the atomic compositions provided in Table 1. Following casting, the slabs were hot rolled through successively smaller roll gaps to produce hot band coils, which were subsequently subjected to cold rolling and annealing cycles until the targeted thickness of approximately 1.4 mm was achieved representing sheet in Step 2 in
Tensile specimens were cut from the sheet via wire EDM. The specimens had two notches, symmetric at about the center of the width and the length as showed in
Tensile properties of the Alloy 2 sheet samples as a function of notch diameter and notch depth are listed in Table 11. Tensile elongation of notched samples ranged from 12.4% to 40.7%, yield strength ranged from 298 to 420 MPa, and ultimate tensile strength ranged from 636 to 1123 MPa. Effect of notch diameter with constant depth of 0.5 mm on tensile properties of the sheet from Alloy 2 is illustrated in
This Case Example demonstrates an increase in tensile elongation of the notched samples from alloys herein with increasing notch diameter at constant depth. In the case of increasing depth, average elongation is shown to be independent of the notch depth (half circle).
SEM fracture analysis was performed on selected notched specimens from Alloy 2 sheet after tensile testing (see Case Example #7). Two samples with notch radius of 1.0 and 6.0 mm were selected for examination (Table 12). The SEM study was conducted using an EVO-60 scanning electron microscope manufactured by Carl Zeiss SMT Inc.
In
This Case Example demonstrates that notch introduction into the sheet material from alloys herein does not cause brittle catastrophic failure. Notched samples after testing have demonstrated ductile fracture.
The alloys herein may be utilized in variety of applications. For example, the alloys herein may be positioned in vehicular frame, vehicle chassis or vehicle panel. In addition, the alloys herein may be utilized for a storage tank, freight car, or railway tank car. Railway tank cars may specifically include tanks, jacketed tanks or tanks with a headshield. Other applications include body armor, metallic shield, military vehicles, and armored vehicle Such applications apply to the alloys produced according to any one of
| Number | Date | Country | |
|---|---|---|---|
| 62527400 | Jun 2017 | US |
