AU2021216446A1

AU2021216446A1 - Improvements in hot band in high strength steel alloys

Info

Publication number: AU2021216446A1
Application number: AU2021216446A
Authority: AU
Inventors: Daniel James Branagan; Kurtis R. CLARK; Grant G. Justice; Scott T. LARISH; Alla V. Sergueeva
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 2020-02-03
Filing date: 2021-02-03
Publication date: 2022-09-22
Also published as: BR112022015099A2; JP2023513091A; EP4100552A4; CN115812108A; EP4100552A1; WO2021159142A1; KR20220156817A; CA3169236A1; MX2022009457A

Abstract

Advanced high strength steel alloys are disclosed with a combination of toughness and a Directional Toughness Ratio (DTR). A combination of yield strength and a Tensile Squareness Ratio (TSR) can be achieved by rolling of the hot band at ambient or identified elevated temperatures.

Description

IMPROVEMENTS IN HOT BAND IN HIGH STRENGTH STEEL ALLOYS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Serial No. 63/001,591, filed March 30, 2020 and U.S. provisional application Serial No. 62/969,262, filed February 3, 2020, both of which are incorporated herein by reference.

Field of Invention

This application deals with a new class of advanced high strength steel alloys with a combination of toughness and a Directional Toughness Ratio (DTR). A combination of yield strength and a Tensile Squareness Ratio (TSR) can be achieved by rolling of the hot band at ambient or identified elevated temperatures.

Background

Toughness, or resistance to fracture, is crucial in applications across many industries. Automotive manufacturers look for materials with high toughness to absorb energy during a crash event. Shipping industries such as rail need materials with high toughness to protect cargo during transport and in the event of a collision or derailment. Materials with high toughness are desirable in these applications to provide and improve efficiency and safety for both the public and cargo.

Toughness as an engineering property can be thought of in a simplified form as the work energy needed to cause failure in a material. The higher the work required to cause failure by a method, the higher the toughness of the material. Toughness in materials is becoming increasingly important across many sectors, especially where tough materials can be used to improve safety. In the automotive industry, high toughness materials are seeing use in so-called crumple zones to reduce the energy that enters the passenger compartment during a collision. Using high toughness materials, gauge thicknesses can be reduced in automobiles in parts where energy absorption is needed to protect passengers, increasing fuel efficiency without compromising safety. These high toughness materials can also be used for road barriers to keep out-of-control vehicles from leaving the roadway or entering the opposing traffic by absorbing energy from the vehicle and safely stopping it. The automotive industry is not alone in the need for high toughness materials, however. The safety of cargo transported overland by rail and on waterways by ships can also be improved with high toughness materials. In recent years, several high-profile incidents where cargo vessels were damaged during collisions or derailments have occurred that have resulted in significant loss of life, property, and cargo. New regulations have been introduced to lessen the probability and impact of such events, and the use of high toughness materials to ensure improved cargo containment is one option available. By increasing the toughness of materials for these shipping containers, cargo can be kept inside the container during such an event and will reduce environmental impact and loss of life or property damage that could result from wayward cargo. High toughness materials therefore provide many industries the opportunity to improve fuel and cargo efficiency while maintaining or improving safety.

Uniform or isotropic toughness in more than one plane or orientation is of great importance in real-world applications. Designers can plan for specific impacts to best use anisotropic toughness in ideal cases, but this is difficult to achieve in an uncontrolled event. During dynamic events such as collisions, it is common for multiple impacts to occur throughout the event. The first impact may start in a favorable manner and result in controlled material deformation. Subsequent impacts in a complex event are likely to occur at different angles of incidence and may be in unfavorable orientations for a given material. These events can be more easily managed in materials where the toughness in different orientations are similar or uniform. If the toughness in different orientations varies considerably for a material, high toughness is unlikely to be realized in a real-world application due to non-ideal impact configurations. For instance, layered or laminated structures have been developed to achieve Charpy V-notch toughness that is greater than 400 J when tested in favorable orientations such as perpendicular to the layered plane. When tested in other orientations, however, the toughness can suffer greatly, including instances where the toughness is often much less (for example 1/5) of the favorable orientation. During a simple impact event such as a controlled experiment in a laboratory where the orientation is favorable, these materials would be expected to perform well. However, in real-world applications with uncontrolled and unfavorable impact loads, the effective toughness could be low or nearly zero. By using materials with more uniform toughness in multiple orientations, a non-ideal impact event would result in diminished toughness as compared to a controlled experiment, but these materials are likely to be considerably more effective at energy absorption than materials with less uniform toughness.

Multicomponent systems also pose potential complications for realizing high toughness in real-world applications. The toughness of a multicomponent system is a sum of the energy absorption properties in the system, and the toughness of different pieces of a multicomponent system are often different. A weak point will always exist within any multicomponent system, and this weak point will typically fail before the remainder of the system. During a failure event, the other components in the system will deform and absorb energy until the system is compromised by the failure of the weak point. In these multicomponent systems, the full toughness of a material will not be realized because the individual part may only see a partial strain until system failure, for example at 10, 20% or 30% strain. A material that achieves high toughness as a result of high tensile elongation and moderate yield strength therefore may not contribute adequate energy absorption in the system during such an event. This is because a large portion of high toughness, moderate yield material energy absorption, likely occurs at strains greater than the failure strain of low toughness or low ductility materials which may be in the 10 to 30% strain range. Alternately, a material with similar toughness but with higher yield strength, ultimate tensile strength and possibly lower elongation might be expected to contribute more energy absorption prior to system failure in the 10 to 30% strain range. High yield and ultimate tensile strength are therefore desirable in multicomponent systems to provide high energy absorption as a result of high loads at low strain prior to the failure of the system at the weak point. Materials with lower yield and ultimate tensile strength but high tensile elongation can be modified to absorb more energy earlier in their deformation through methods that result in increased yield and ultimate tensile strength including but not limited to rolling at temperatures below their recrystallization and recovery temperatures. Through these methods, materials can be altered to better meet the needs of multicomponent systems. Advanced High Strength Steels (AHSS’s) are those classes of materials whose mechanical properties are superior to the conventional steels. Conventional mild steel has a relatively simple ferritic microstructure; it typically has low carbon content and minimal alloying elements, is readily formed, and is especially sought for its ductility. Widely produced and used, mild steel often serves as a baseline for comparison of other materials. Conventional low- to high- strength steels include IF (interstitial free), BH (bake hardened), and HSLA (high-strength low- alloy). These steels generally have yield strength of less than 550 MPa and ductility that decreases with increased strength. Higher strength steels are more complex and include such grades as dual phase (DP), complex phase (CP) and transformation induced plasticity (TRIP) steels. High strength steels are those which exhibit a tensile strength of 750 MPa or greater. The development of advanced high strengths steel has been a challenge since increased strength (i.e. > 750 MPa) often results in reduced ductility, cold formability, and toughness.

Fracture toughness is a material specific parameter that quantifies a material’s resistance to fracture under specific loading conditions. However, fracture toughness is difficult to measure for many engineering materials due to the test requirements in order to achieve proper fracture without buckling or other unwanted plastic deformation. As a result, other methods of quantifying toughness are regularly employed that are more appropriate to the desired final application and simulate real-world fracture conditions. Calculating the area under a tensile stress - tensile strain curve is one method and provides an approximation of the energy required to fracture a specimen under uniaxial tensile loading with a relatively constant strain rate. Dynamic impact tests are another method used to quantify toughness, wherein rapid, dynamic strains are applied to the material during the test. These tests encourage fracture by limiting the time available for plastic deformation. Charpy v-notch testing is a commonly used technique to measure a material’s resistance to fracture with a crack that was introduced prior to the test. Drop impact tests can also be used to gauge toughness, as a measurement of the material’s ability to resist fracture caused by a moving mass without a previously introduced crack. Results from toughness tests including but not limited to those mentioned previously do not provide a material parameter unlike fracture toughness testing, but rather a value specific to that test, material, and loading condition. The values from each test can be compared to other materials in the same test when needed for materials selection. Typically, steel grades with high ductility measured as a total elongation during tensile tests also demonstrate high toughness during impact testing. However, high ductility comes with decrease in strength characteristics including yield strength.

Summary

The present invention is directed at a method to achieve a combination of properties including toughness and a directional toughness ratio (DTR) in hot band from high strength steel alloys comprising: a. supplying a metal alloy comprising at least 65 at.% Fe with Mn, Cr, Si, and C, and optionally Ni and/or Cu; melting said alloy, cooling at a rate of < 250 K/s, and solidifying to a thickness of 25.0 mm up to 500 mm; b. processing said alloy by heating and reducing said thickness by rolling the metal alloy in step (a) in a selected direction to form a sheet with thickness of 10.0 mm to 20.0 mm, optionally subjecting said alloy sheet to a temperature from 600 °C up to but not including Tm where T_m is a melting point of said alloy, to produce said alloy sheet with a total elongation, El from 30 to 75%, a yield strength at 0.2% offset, Yl, from 250 to 525 MPa, an ultimate tensile strength, Ul, from 750 to 1400 MPa and a Tensile Squareness Ratio, TSR1, from 0.65 to 0.90, wherein:

(1) a V-notch Charpy sample cut from said alloy sheet absorbs impact energy of 150 J to 850 J; and

(2) the impact energy absorbed by a V-notch Charpy sample cut from said alloy sheet where said sample is notched perpendicular to a longitudinal-normal plane of the sheet divided by the impact energy absorbed by a V-notched Charpy sample cut from said alloy sheet where said sample is notched perpendicular to a transverse-longitudinal plane of said sheet provides a directional toughness ratio (DTR) from 0.8 to 1.5.

Additionally, a method to achieve a combination of properties including yield strength and a tensile squareness ratio (TSR) in hot band from steel alloys involving: a. supplying a metal alloy comprising at least 65 at.% Fe with Mn, Cr, Si, and C, and optionally Ni and/or Cu; melting said alloy, cooling at a rate of < 250 K/s, and solidifying to a thickness of 25.0 mm up to 500 mm; b. processing said alloy by heating and reducing said thickness to form a sheet with thickness of 10.0 mm to 20.0 mm, optionally subjecting said alloy sheet to a temperature from 600 °C up to but not including Tm, to produce said alloy sheet with a total elongation, El from 30 to 75%, a yield strength at 0.2% offset, Yl, from 250 to 525 MPa, an ultimate tensile strength, Ul, from 750 to 1400 MPa and a Tensile Squareness Ratio, TSR1, from 0.65 to 0.90; c. subjecting said alloy sheet to a rolling reduction in thickness at: (1) at a first temperature range T1 of 15 C to < 50 C with a reduction in thickness of said sheet in step (b) of 1 to 10%; or (2) at a second temperature range of 50 °C to < 600 °C with a reduction of thickness of said sheet in step (b) of 10% to 40% to produce an alloy sheet having yield strength Y2>Y1, and Tensile Squareness Ratio TSR2>TSR1.

Brief Description of the Drawings

FIG. 1 Summary of steps towards alloys herein to achieve novel combinations of properties including toughness and a Directional Toughness Ratio.

FIG. 2 Summary of steps towards alloys herein to achieve novel combinations of properties including yield strength and Tensile Squareness Ratio.

FIG. 3 Examples of engineering stress - strain curves with equal area under the curve

(SA=SB) representing energy absorption through entire tensile test. Note that Material 1 and Material 2 have different behavior during the testing and the Material 2 has higher energy absorption during initial straining.

FIG. 4 Example engineering stress - strain curve showing two regions; A) Toughness calculation based on product of UTS multiplied by strain at UTS (rectangular region light shading), and B) Toughness calculation based on area under the curve up to UTS (dark shading). Note that the TSR is defined as a ratio of area with B/A.

FIG. 5 Diagram of V-notch Charpy sample orientations taken from the alloy sheet, to determine a Directional Toughness Ratio (DTR), which was calculated as the impact energy absorbed by a V-notch Charpy sample cut from said alloy sheet where said sample is notched perpendicular to a longitudinal-normal plane of the sheet divided by the impact energy absorbed by a V-notched Charpy sample cut from said alloy sheet where said sample is notched perpendicular to a transverse- longitudinal plane of said sheet.

FIG. 6 Schematic illustration of the Charpy V-notched sample. Note that dimensions are in mm unless otherwise indicated.

FIG. 7 An image of the unbroken Alloy 80 sample in L-N orientation after Charpy testing.

FIG. 8 SEM image of the fracture surface of Alloy 66 sample in L-N orientation after Charpy testing.

FIG. 9 SEM image of the fracture surface of Alloy 66 sample in L-T orientation after Charpy testing.

FIG. 10 SEM image of the fracture surface of Alloy 80 sample in L-T orientation after Charpy testing.

FIG. 11 SEM image of the fracture surface of Alloy 84 sample in L-N orientation after Charpy testing.

FIG. 12 SEM image of the fracture surface of Alloy 84 sample in L-T orientation after Charpy testing.

FIG. 13 SEM micrograph of fracture surface of hot band from Alloy 2 before annealing. FIG. 14 SEM micrograph of fracture surface of hot band from Alloy 2 after annealing at 600°C for 10 min.

FIG. 15 SEM micrograph of fracture surface of hot band from Alloy 3 before annealing. FIG. 16 SEM micrograph of fracture surface of hot band from Alloy 3 after annealing at 600°C for 10 min.

FIG. 17 Types of material fracture behavior during Instrumented Charpy testing; a) Type I, b) Type II, c) Type III, and d) Type IV.

FIG. 18 Force-displacement curve for Alloy 18. Note that Alloy 18 shows Type IV behavior.

FIG. 19 Force-displacement curve for Alloy 32. Note that Alloy 32 shows Type IV behavior.

FIG. 20 Force-displacement curve for Alloy 37. Note that Alloy 37 shows Type IV behavior.

FIG. 21 Force-displacement curve for Alloy 44. Note that Alloy 44 shows Type IV behavior.

FIG. 22 Yield strength as a function of reduction during rolling at ambient temperature of hot band from Alloy 66. FIG. 23 Yield strength as a function of reduction during rolling at ambient temperature of hot band from Alloy 80.

FIG. 24 Yield strength as a function of reduction during rolling at ambient temperature of hot band from Alloy 84.

FIG. 25 Tensile Squareness Ratio as a function of reduction during rolling at ambient temperature of hot band from Alloy 66.

FIG. 26 Tensile Squareness Ratio as a function of reduction during rolling at ambient temperature of hot band from Alloy 80.

FIG. 27 Tensile Squareness Ratio as a function of reduction during rolling at ambient temperature of hot band from Alloy 84.

FIG. 28 Yield strength as a function of reduction during rolling of hot band from Alloy 66 at 550°C.

FIG. 29 Yield strength as a function of reduction during rolling of hot band from Alloy 80 at 600°C.

FIG. 30 Yield strength as a function of temperature during rolling of hot band from Alloy 84 with 10% reduction.

FIG. 31 Tensile Squareness Ratio as a function of reduction during rolling of hot band from Alloy 66 at 550°C.

FIG. 32 Tensile Squareness Ratio as a function of reduction during rolling of hot band from Alloy 80 at 600°C.

FIG. 33 Tensile Squareness Ratio as a function of temperature during rolling of hot band from Alloy 84 with 10% reduction.

FIG. 34 Force-displacement curve for hot band from Alloy 88 with thickness of 11.9 mm. Note that it shows Type IV behavior.

FIG. 35 Force-displacement curve for hot band from Alloy 88 after rolling at ambient temperature with 3% reduction. Note that it shows Type IV behavior. FIG. 36 Force-displacement curve for hot band from Alloy 88 after rolling at ambient temperature with 9% reduction. Note that it shows Type IV behavior.

FIG. 37 Force-displacement curve for hot band from Alloy 88 with thickness of 17.5 mm. Note that it shows Type IV behavior.

FIG. 38 Force-displacement curve for hot band from Alloy 88 after rolling with 20% reduction at 550°C. Note that it shows Type IV behavior.

FIG. 39 Force-displacement curve for hot band from Alloy 88 after rolling with 40% reduction at 550°C. Note that it shows Type IV behavior.

Detailed Description of Preferred Embodiments

Alloys herein can be produced in a sheet or plate form by different methods of casting including but not limited to continuous casting, thin slab casting, thick slab and bloom casting with achievement of advanced property combinations by subsequent hot rolling and optionally heat treating. Additional rolling at ambient temperature or rolling at identified elevated temperature can be applied. FIG. 1 and FIG. 2 illustrate a summary of steps towards alloys herein to achieve novel combinations of properties including toughness, yield strength, a Tensile Squareness Ratio (TSR), and a Directional Toughness Ratio (DTR) in a hot band with thickness of 2 mm to 20 mm.

FIG. 1 illustrates a summary of steps towards alloys herein to achieve a combination of properties in high strength steel including toughness and a Directional Toughness Ratio (DTR). Reference to high strength steel are those steel that exhibit a tensile strength of 750 MPa or greater. In addition, the method applies to a hot band steel, which may be understood as a sheet of steel that has been hot-rolled to reduce the sheet thickness.

Accordingly, in Step 1 in FIG. 1, the starting condition is to supply a metal alloy Fe, Mn, Cr, Si and C, and additionally at least one element selected from Ni and Cu. The alloy chemistry is melted, cooled at a rate of < 250 K/s, and solidified to a thickness of 25 mm and up to 500 mm. The casting process can be done in a wide variety of processes including ingot casting, bloom casting, continuous casting, thin slab casting, thick slab casting, belt casting etc. Preferred methods would be continuous casting in sheet or plate form by thin slab casting or thick slab casting.

Step 2 in FIG. 1 corresponds to a hot band sheet or plate from the said alloy with thickness from 10.0 to 20.0 mm. To produce alloys herein in a hot band form, hot rolling in a selected direction is applied to a cast product (slab, bloom, etc.). As an example, consider thick slab casting as one process route to get to hot band product. The alloy would be cast going through a water cooled mold typically in a thickness range of 150 to 350 mm in thickness and typically processed through a roughing mill hot rolling into a transfer bar slab of 25 to 150 mm in thickness and through the finishing mill into a hot band with thickness of 10.0 to 20.0 mm. More preferably, the thickness of the alloy in Step 2 may be 10.0 mm to 15.0 mm, or even more preferably, the alloy thickness may be 10.0 mm, 11.0 mm, 12.0 mm, 13.0 mm, 14.0 mm, 15.0 mm, 16.0 mm, 17.0 mm, 18.0 mm, 19.0 mm or 20.0 mm.

Another example would be to preferably process the cast material through a thin slab casting process. In this case, after casting typically forms 25 to 150 mm in thickness by going through a water-cooled mold, the newly formed slab goes directly to hot rolling and the strip is rolled into hot band coils with typical thickness from 10.0 to 20.0 mm. 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 hot rolling. Thin slab casting is another process route allowing to get to hot band product with thickness from 10.0 to 20.0 mm. By heating and reducing said thickness to form a thickness of 10.0 to 20.0 mm, a relatively ductile micro structure develops providing a total elongation from 30 to 75% which exhibits a microvoid coalescence mechanism during fracture. Note that microvoid coalescence is a ductile failure mechanism involving nucleation, growth, and coalescence of microvoids and is preferable to other brittle fracture modes such as intergranular fracture or transgranular cleavage. Optionally, a hot band product can be subjected to temperature from 600°C to but not including T_m, where T_m is a melting point of said alloy, by annealing. Preferably, a hot band from alloys herein has a total elongation (El) from 30 to 75%, a yield strength (Yl) from 250 to 525 MPa, and a tensile strength (Ul) from 750 to 1400 MPa.

Some methods for material toughness evaluation are based on tensile testing, which is one of the most widely used methods for mechanical properties evaluation and generally performed by applying a tensile load to a sample with a reduced section by a moving crosshead until the sample fails. The displacement rate of the crosshead in tensile testing is generally kept constant or near constant, resulting in a narrow range of strain rates throughout the test. Tensile testing can provide a measure of toughness by calculating the integral of the engineering stress - engineering strain curve (an area under the curve) and corresponds to the work required to break the sample in tension (FIG. 3). Material behavior during tensile testing in Step 2 in FIG. 1 correspond to Material 1 in FIG. 3 by multiplying the ultimate tensile strength by the total elongation (strength-ductility product) and for the alloys herein was determined to vary from 25,000 to 80,000 MPa%. The area under the stress- strain curve corresponding to the work required to strain the material to 10% (Slo_.i) is in a range from 4,500 to 8,500 MPa%, to strain to 20% (SI0_.2) is in a range from 10,500 to 18,500 MPa%, and to strain to 30% (SI0_.3) is in a range from 17,500 to 27,000 MPa%.

Another toughness estimation method is to look at uniform elongation only (prior to necking). In Fig. 4 this is shown by the Region A which is rectangular and bounded by a dotted line and is defined specifically as the product of UTS multiplied by strain at UTS. The alloys herein exhibit region A toughness values from 20,000 to 65,000 MPa%. The area under the stress strain curve up to the UTS point is represented by the Region B in Fig. 4. The ratio of B/A, defined as the Tensile Squareness Ratio (TSR), provides a measure of the material toughness of the material achieved based on what is possible for the material. Thus, the closer the TSR to 1.0, the higher toughness achieved in the material compared to its potential value. A Tensile Squareness Ratio (TSR1) in alloys herein was calculated in a range from 0.65 to 0.90.

It should be noted that the alloy sheet produced in step (2) is such that it may be positioned as all or part of a storage tank, freight car, railway tank car, vehicular frame, vehicular chassis, vehicular panel and/or the alloy sheet can be utilized in a battery exo-skeleton (i.e. the external skeleton that protects the battery from external damage), battery tray (i.e. a protective structure which may consist of upper and lower halves to prevent water or other corrosive agents from contacting the battery and to provide for improved thermal management), or battery cage (i.e. a structure designed to confine and protect individual batteries or multiple batteries). Accordingly, the alloy sheet can be configured and utilized as all or a portion of any one of these aforementioned applications. Moreover, the impact energy absorbed by a V-notch in the sheet in the range of 150 J to 850 J is such that it would occur on any such alloy sheet positioned and utilized for these applications.

In Step 3 in FIG. 1, a hot band from alloys herein is subjected to impact in one or multiple stages until fracture occurs with recording of energy absorption (J). Hot band toughness from said alloy can be evaluated by Charpy V-notch impact testing. Charpy impact testing is performed by the dynamic loading of a sample by a swinging hammer starting from a known height and distance from the center of rotation. The ends of the samples in Charpy impact testing are free and the loading of the sample is similar to a three-point bend test. The total energy of the moving hammer is known, and the energy lost in the impact event with the sample can be measured by the rotation angle of the hammer after impact. In Charpy V-notch testing the sample has a pre-machined stress concentration point at the V-notch tip which helps encourage crack nucleation. In this test, the hammer strikes the side opposite the machined notch. Charpy V-notch impact testing measures the work required to plastically deform the sample as well as crack nucleation and propagation. Uniformity of the toughness in the sheet or plate from the alloys herein may be determined by a Directional Toughness Ratio (DTR). As may be appreciated, the alloys sheet herein may be described, as illustrated in FIG. 5, to have a longitudinal-normal plane, which as shown is the plane that otherwise defines the extended edge portion of the sheet. The longitudinal direction coincides with the rolling direction of the sheet. In addition, the sheet may include a transverse-longitudinal plane, which as shown is a plane that otherwise defines the upper or lower surface of the sheet. One may then cut a sample from said sheet and provide a notch that is perpendicular to the longitudinal-normal plane of the sheet and measure the Charpy impact for such first sample (L- N). One may then also cut a sample from said sheet and provide a notch that is perpendicular to the transverse-longitudinal plane of the sheet and measure the Charpy impact for such second sample (L-T). The value of DTR is then determined by dividing the Charpy impact result for such first sample (L-N) by the Charpy impact result for the second sample (L-T). It should be appreciated that in FIG. 5, the identification of the portion L-N and L-T are samples that are taken from the sheet alloy herein having a thickness of 10.0 mm to 20.0 mm. Thus, the closer the TSR to 1.0, the more uniform toughness achieved in the alloy. Hot band from alloys herein has Directional Toughness Ratio (DTR) from 0.8 to 1.5. Preferably, a hot band from alloys herein has Charpy V-notch toughness (Jl) from 150 J to 850 J.

FIG. 2 illustrates a summary of steps towards alloys herein to achieve novel combinations of properties including yield strength and a Tensile Squareness Ratio (TSR) in a hot band from alloys herein after rolling at ambient or intermediate temperature. Step 1 and Step 2 are the same as in FIG. 1. In Step 3 in FIG. 2, a hot band from alloys herein is subjected to rolling at a first or second temperature range (described herein) when the sheet is passed through a set of rolls to further reduce the sheet thickness. Rolling can be accomplished via a number of means, with mills of a myriad of configurations, including but not limited to reversing mills, tandem mills, and Sendzimir mills. During this process, the hot band thickness is reduced by plastically deforming the material utilizing the deformation mechanisms that are available in the material at the first temperature range or second temperature range leading to a change in material properties and behavior. Rolling of hot band from alloys herein preferably occurs at a first ambient temperature range (Tl) in a range from 15 °C to < 50 °C with reduction from 1 to 10% or at a second higher temperature range (T2) of 50 to 600°C with reduction in thickness from 10 to 40%. In either case, it results in the formation of the metal alloy sheet with an increase in yield strength of the material where Y2>Y 1 leading to higher Tensile Squareness Ratio TSR2>TSR1. TSR2 is calculated to be in a range from 0.75 to 0.95 for the alloys herein. Material behavior during tensile testing in Step 3 in FIG. 2 corresponds to Material 2 in FIG. 3. To further define the shape of stress strain curve and the toughness at intermediate points, three applied strain levels are defined and shown in FIG. 3 at strain levels of 10%, 20%, and 30%. The area under the stress-strain curve related to the work require to strain the material to 10% (S2o_.i > Slo_.i) is in a range from 5,500 to 10,500 MPa%, to strain to 20% (S2o_.2 > S I0_.2) is in a range from 13,000 to 21,000 MPa%, and to strain to 30% (S2o_.3 > SI0_.3) is in a range from 21,100 to 32,500 MPa%.

It should be noted that the alloy sheet produced in step (3) is such that it may be positioned as all or part of a storage tank, freight car, railway tank car, vehicular frame, vehicular chassis, vehicular panel and/or the alloy sheet can be utilized in a battery exo-skeleton, battery tray, or battery cage. Accordingly, the alloy sheet can be configured and utilized as all or a portion of any one of these aforementioned applications.

Main Body Alloys

The chemical composition of the alloys herein is shown in Table 1, which provides the preferred atomic ratios utilized.

Table 1 Chemical Composition Of Alloys (Atomic %)

As it can be seen from Table 1, preferably, in the alloys herein, Fe is present at a level of greater than 65 at. % with Mn, Cr, Si, and C and one or two elements from the group of Ni and Cu to provide a formulation of elements that totals 100 atomic percent. More preferably, the alloys herein can be described as comprising, consisting essentially of, or consisting of the following elements at the indicated atomic percent (when present): Fe (65.0 to 80.0 at.%), Mn (9.5 to 17.5 at.%), Cr (1.0 to 10.0 at.%), Si (1.0 to 5.5 at.%), and C (0.5 to 1.5 at.%), and optionally Ni (0.2 to 4.0 at.%), and Cu (0.1 to 2.5 at.%). The total level of impurities of other elements is in the range of 0 to 5,000 ppm. Accordingly, for an individual element to be defined as an impurity the total amount of the specific element is < 1,000 ppm. The level of all such selected elements may then be in combination be present to account for the 5,000 ppm impurity, such that the total of all elements present (selected elements and impurities) is 100 atomic percent. The alloys herein were processed into a laboratory sheet by processing of laboratory slabs. Laboratory alloy processing is developed to mimic closely the commercial sheet production by continuous casting and hot rolling. Annealing might be applied depending on targeted properties.

Laboratory Slab Casting

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. Impurities can be present at various levels depending on the feedstock used. Impurity elements would commonly include the following elements; Co, Al, N, P, Ti,, W, Mo, Nb, V, Ga, Ge, Sb, Zr, O, Sn, Ca, B, and S which if present would be in the range from 0 to 5,000 ppm (parts per million) (0 to 0.5 wt%) at the expense of the desired elements noted previously. Preferably, the level of impurities is controlled to fall in the range of 0 to 3,000 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.

Physical Properties of Cast Alloys

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 1510°C (maximum temperature limit for the used DSC equipment) 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 1369 to 1469°C as listed in Table 2. Preferred embodiments for the alloys herein are a solidus temperature from 1350°C to 1450°C, a liquidus temperature from 1400°C to 1500°C, and a liquidus to solidus gap from 40°C to 100°C. Thermal analysis provides information on maximum temperature for the following hot rolling processes that varies depending on alloy chemistry.

Table 2 Thermal Analysis Of Alloys

The density of the alloys herein was measured on samples from 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 3 and was found to be in the range from 7.74 to 7.91 g/cm³. The accuracy of this technique is ± 0.01 g/cm³.

Table 3 Density Of Alloys Laboratory Processing Into Hot Band Through Hot Rolling for Tensile Testing

To measure tensile properties of the material, it was hot rolled to thinner gauges since testing capability do not allow tensile testing of samples with 10 mm thickness due to load cell limitations. Slabs from Alloy 1 through Alloy 44 in Table 1 were hot rolled to final thickness of ~4 mm. Prior to hot rolling, laboratory slabs were preheated in a Lucifer EHS3GT-B18 furnace. The furnace set point varied 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 were hot rolled for 5 to 10 passes though the mill before being allowed to air cool. Final thickness after hot rolling ranges from 3.89 to 4.24 mm. Although this tensile testing was on such final thicknesses due to load limitations, it is reasonable to conclude that such tensile properties would be present at the range of 10.0 mm to 20.0 mm, since the hot band structure becomes sufficiently homogenized in this range.

Tensile specimens were cut from laboratory hot band using wire EDM (electrical discharge machining). Tensile properties were measured on an Instron mechanical testing frame (Model 3369) with hydraulic grips, utilizing Instron’ s Bluehill control and analysis software. Note that the load limits for the 3369 test frame is 150 kN with the hydraulic grips limited to a load of 120 kN. Due to the high strengths of the alloys within and load limit considerations, the maximum sample thickness that can be tensile tested until failure is 7 mm in thickness. To get tensile data then processing was continued to produce hot band of 4 mm thick and results for hot band samples (three to four specimens for each alloy) with thickness of 4 mm are listed in Table 4 including area under the stress - strain curve. The ultimate tensile strength values of the annealed sheet from alloys herein are in a range from 784 to 1218 MPa, the yield strength at 0.2% offset varies from 257 to 391 MPa, the total elongation recorded in the range from 36.6 to 72.1%, and area under tensile stress-strain curve is in a range from 29,272 to 61,055 MPa%.

Strength ductility product is calculated as UTS multiplied by total tensile elongation varies from 41,800 to 73,600 MPa%. A tensile squareness ratio (TSR) is determined as the area under stress - strain curve up to UTS divided by the product of UTS multiplied by strain at UTS (FIG. 4) and the calculated values are in a range from to 0.70 to 0.85. An area under tensile curve for alloys herein after straining to 10%, 20%, and 30% is shown in Table 5. The area under the stress- strain curve corresponding to the work required to strain the material to 10% (S10.1) is in a range from 4,791 to 5,973 MPa%, to strain to 20% (S10.2) is in a range from 10,949 to 13,556 MPa%, and to strain to 30% (S 10.3) is in a range from 17,933 to 23,256 MPa%. Note that the Table 4 and Table 5 properties correspond to Step 2 in FIG. 1 and FIG. 2.

Table 4 Tensile Properties Of Hot Band Sheet (~4 mm Thick)

Table 5 Area Under Tensile Curve At Strain (~4 mm Thick Hot Band)

Slabs from Alloy 45 through Alloy 88 in Table 1 were hot rolled to final thickness of ~2.5 mm. Prior to hot rolling, laboratory slabs were preheated in a Lucifer EHS3GT-B 18 furnace. The furnace set point varied 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 were hot rolled for 5 to 10 passes though the mill before being allowed to air cool. Final thickness ranges after hot rolling from 2.37 to 2.60 mm. The hot band in this thickness range is expected to have similar properties in the thickness range of 10 to 20 mm but hot band in this reduced thickness range was much faster to cut through wire EDM into tensile specimens.

Tensile properties were measured on an Instron mechanical testing frame (Model 3369) with hydraulic grips, utilizing Instron’ s Bluehill control and analysis software. Note that the load limits for the 3369 test frame is 150 kN with the hydraulic grips limited to a load of 120 kN. Due to the high strengths of the alloys within and load limit considerations, the maximum sample thickness that can be tensile tested until failure is 7 mm in thickness. Tensile testing results for hot band with thickness of 2.5 mm are listed in Table 6 including area under the stress - strain curve. The ultimate tensile strength values of the sheet from alloys herein are in a range from 902 to 1383 MPa, the yield strength at 0.2% offset (a valued determined by drawing a parallel line on the initial stress strain curve at a 0.2% offset with the resulting point of intersection recorded) varies from 267 to 504 MPa, the total elongation recorded in the range from 34.7 to 65.3 %, and area under tensile stress-strain curve is in a range from 30,497 to 64,399 MPa%. Strength ductility product is calculated as UTS multiplied by total tensile elongation varies from 41,755 to 79,325 MPa%. A tensile squareness ratio (TSR) is determined as the area under stress - strain curve up to UTS divided by the product of UTS multiplied by strain at UTS (FIG. 4) and the calculated values are in a range from to 0.66 to 0.89. An area under tensile curve for alloys herein after straining to 10%, 20%, and 30% is shown in Table 7. The area under the stress-strain curve corresponding to the work required to strain the material to 10% (S lo_.i) is in a range from 5,521 to 6,861 MPa%, to strain to 20% (SI 0_.2) is in a range from 13,676 to 15,310 MPa%, and to strain to 30% (SI0_.3) is in a range from 22,332 to 25,777 MPa%. Note that the Table 6 and Table 7 properties correspond to Step 2 in FIG. 1 and FIG. 2.

Table 6 Tensile Properties Of Hot Band Sheet (-2.5 mm Thick)

Table 7 Area Under Tensile Curve At Strain (~2.5 mm Thick Hot Band)

Laboratory Processing Into Hot Band Through Hot Rolling for Toughness Testing

Prior to hot rolling, laboratory slabs were preheated in a Lucifer EHS3GT-B18 furnace. The furnace set point varied 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 were hot rolled for 5 to 10 passes though the mill before being allowed to air cool. Final thickness ranges after hot rolling is about 12 mm.

Materials toughness was measured by Charpy V-notch testing. Charpy V-notch samples were machined by wire EDM from hot rolled sheet. Charpy V-notch samples are machined in L- T (sample length in rolling direction, notch in transverse direction) and L-N (sample length in rolling direction, notch in normal direction to rolled surface) orientations as shown in FIG. 5.

Charpy V-notch samples were cut in accordance with ASTM E23-12c (10 mm x 55 mm x 10 mm) thickness with a centered 45° V-notch of 0.25 mm radius, 2 mm in depth with a surface finish Ra of less than 2.0 pm on notch and strike face). An example of the Charpy V-notch specimen before testing and its schematic illustration are shown in FIG. 6. Charpy V-notch samples are mounted using self-centering tongs to ensure the samples are centered on the anvil. Testing was done by using the Satec Systems S1-1K3 Pendulum Impact Tester. The arm of the Impact Tester is set to the high latch position with 66.6 lb weights configured for indicating dial maximum reading of -400 J. The latch is released and after contacting the sample, the reading of energy absorbed by the sample is recorded in joules.

Testing results are shown in Table 8. Absorbed energy values during Charpy V-notch testing of alloys herein are in a range from 154 to 407 J in hot band condition. Based on the test results, Directional Toughness Ratio (DTR) was calculated as the impact energy absorbed by a V- notch Charpy sample cut from said alloy sheet where said sample is notched perpendicular to a longitudinal-normal plane of the sheet divided by the impact energy absorbed by a V-notched Charpy sample cut from said alloy sheet where said sample is notched perpendicular to a transverse-longitudinal plane of said sheet. The DTR (L-N/L-T) for alloys herein varies from 0.93 to 1.44. Note that the Table 8 properties correspond to Step 3 in FIG. 1.

Table 8 Charpy V-Notch Toughness Of Hot Band (10 mm Thick)

Case Examples

Case Example #1 Uniformity of Hot Band Toughness

Alloy 66, Alloy 80 and Alloy 84 hot band with a thickness of >10 mm was used for impact toughness testing. Laboratory slab with thickness of 80 mm was cast from alloys according to the atomic ratios provided in Table 1 and laboratory processed by hot rolling as described in the Main Body section of the current application down to a thickness of ~ 12 mm.

V-notch Charpy samples were EDM cut from the laboratory produced hot band in different orientations as shown in FIG. 5. V-notch Charpy samples used for impact testing were cut from the sheet in the hot rolled condition. The impact testing was conducted using an Instron SI- IB Charpy impact tester. The V-notch Charpy impact energy varies from 175 to 372 J and results are listed in Table 9. Based on the test results, two types of a Directional Toughness Ratio (DTR) were calculated as the ratio between toughness in the normal orientation of the Charpy impact in respect to hot band surface and toughness in the transverse orientation of the Charpy impact. For samples with longitudinal orientation, DTR (L-N/L-T) varies from 1.09 to 1.18. Note that the Table 9 properties correspond to Step 3 in FIG. 1.

Fractured specimens from each alloy in each direction were mounted in a Zeiss MA-10 Scanning Electron Microscope (SEM) for examination of the fracture surface. Note than sample from Alloy 80 in L-N direction did not break during testing and shown in FIG. 7. Micrographs of the fracture surface are shown for samples from Alloy 66 in FIG. 8 and FIG. 9, for samples from Alloy 80 in FIG. 10 and for samples from Alloy 84 in FIG. 11 and FIG. 12. Microvoid coalescence was observed in all specimens in all four orientations indicating ductile fracture.

Table 9 V-Notch Charpy Impact Energy Of Hot Band In Different Orientations This Case Example demonstrates that a hot band from alloys herein has similar impact toughness in different orientations with a Directional Toughness Ratio (DTR) from 1.09 to 1.18. Ductile fracture exhibiting a microvoid coalescence occurred in each orientation.

Case Example #2 Hot Band Toughness Improvement by Annealing

Charpy test specimens were cut by wire EDM from the hot rolled material from selected alloys listed in Table 10. Specimens were cut with two orientations (L-N and L-T) as shown in FIG. 5. The test specimens are then wrapped in foil and placed in a pre -heated furnace at 600°C. Argon gas is injected into the furnace during the annealing process. After ten minutes the samples are removed from the furnace and placed under a fan to cool. Specimens are then cleaned with a Scotch-Brite pad and wire brush prior to testing.

Testing results are shown in Table 10. Absorbed energy values during Charpy V-notch testing of alloys herein are in a range from 258 to 407 J in hot rolled sheet after annealing. Based on the test results, a Directional Toughness Ratio (DTR) was calculated as the ratio between toughness in the normal orientation of the Charpy impact in respect to hot band surface and toughness in the transverse orientation of the Charpy impact (FIG. 5). The DTR (F-N/F-T) for alloys herein varies from 0.91 to 1.28. Note that the Table 10 properties correspond to Step 3 in FIG. 1.

Table 10 Charpy V-Notch Toughness Of Hot Band (10 mm Thick) After Annealing

This Case Example demonstrates that Charpy V-notch toughness of the hot band from alloys herein can be improved by annealing with high toughness in a range from 258 to 407 J and a Directional Toughness Ratio (DTR) from 0.91 to 1.28.

Case Example #3 Fracture Surface Analysis after Charpy Testing (before and after annealing)

To evaluate the impact toughness of hot band material, fracture surface of Charpy samples were examined by SEM. The V-notch Charpy samples were EDM cut from the Alloy 2 and Alloy 3 hot band. Half of the samples were annealed at 600°C for 10 m. Charpy testing was conducted on samples before and after the heat treatment. SEM analysis of the fracture surface was done using an EVO-MAIO scanning electron microscope manufactured by Carl Zeiss SMT Inc. Fracture surface of the hot band samples before and after annealing is shown in FIG. 13 though FIG. 16 for Alloy 2 and Alloy 3.

The fracture surface in the V-notch Charpy sample from Alloy 2 before annealing is shown in FIG. 19. Ductile fracture characteristic of microvoid coalescence is seen in the fracture surface. After the annealing at 600°C for 10 m, the fracture is also characteristic of ductile type (FIG. 17). Even though the fracture toughness is improved after annealing, ductile fracture dominates in both cases. It suggests that higher energy absorption after the heat treatment might be caused by structure relaxation from the annealing, but the fracture surface does not show an obvious difference. Fracture surface before and after annealing of Alloy 3 is shown in FIG. 15 and FIG. 16, respectively. Similar to Alloy 2, ductile fracture is seen before and after annealing in Alloy 3.

This Case Example shows that the fracture of the hot band material is of ductile nature exhibiting a microvoid coalescence fracture mechanism.

Case Example #4 Instrumented Charpy Testing

Alloy 18, Alloy 32, Alloy 37 and Alloy 44 hot band with a thickness of -10 mm was used for instrumented Charpy testing. Laboratory slab with thickness of 80 mm was cast from alloys according to the atomic ratios provided in Table 1 and laboratory processed by hot rolling as described in the Main Body section of the current application down to a thickness of - 10 mm.

Standard V-notch Charpy specimens were cut by wire EDM and tested, recording the energy absorbed. The results are listed in Table 11. Absorbed energy values during Charpy V- notch testing of alloys herein are in a range from 262 to 424 J with Directional Toughness Ratio (DTR) from 0.98 to 1.14. Note that the Table 11 properties correspond to Step 3 in FIG. E The Charpy machine used is instrumented with a small force sensor to record load and an encoder to record hammer velocity. Based on material response to impact, its fracture behavior can be represented by four types of fracture behavior (FIG. 17). Type I behavior is characterized as linear elastic response only. Type II behavior is elastic plastic with unstable cleavage failure without crack extension. Type III fracture behavior is elastic plastic with unstable cleavage failure after stable crack extension. Type IV is elastic plastic with stable crack extension. Note that Type IV is the most ductile fracture type characterized by ductile failure through microvoid coalescense. Force-displacement data were captured in addition to the total energy absorbed and corresponding curves are shown in FIG. 18 through FIG. 21 for each alloy tested. A dynamic fracture toughness (J0_.2) was calculated based on test results and it is listed in Table 11 and varies from 120 to 274 J/cm². The fracture toughness values reported are for notched Charpy specimens. The final fracture toughness values are reported with the subscript “0.2” to indicate the final recommended fracture toughness is the value of Ji_dn at a crack extension of 0.2 mm. The “Idn” subscript indicates Mode I loading, dynamic testing and a notch was used to initiate the crack. The fracture toughness values do not meet the E1820 size requirement Ji_dn<J_max=Boo/10 and should be considered an estimate. Table 11 Instrumented Charpy Test Data

This Case Example demonstrates that a hot band from alloys herein with or without annealing exhibit a ductile Type IV behavior with high toughness in a range from 262 to 424 J and a Directional Toughness Ratio (DTR) from 0.98 to 1.14. Case Example # 5 Toughness of Hot Band from Alloy 88

Laboratory slab with thickness of 80 mm was cast from Alloy 88 according to the atomic ratios provided in Table 1 and laboratory processed by hot rolling as described in the Main Body section of the current application. Charpy V-notch specimens were cut from the hot band before and after cold rolling by wire EDM and tested according to the procedures described in the Main Body of the current application. The results are listed in Table 12. Absorbed energy values during Charpy V-notch testing of alloys herein are in a range from 249 to 298 J with a Directional Toughness Ratio (DTR) of 1.08.

Hot band with a final hot rolled thickness of ~ 11.9 mm was further cold rolled an additional 3% and 9%. Charpy V-notch specimens were cut from the hot band before and after cold rolling by wire EDM and tested according to the procedures described in the Main Body of the current application. The results are listed in Table 12. Absorbed energy values during Charpy V-notch testing of alloys herein are in a range from 96 to 158 J with Directional Toughness Ratio (DTR) from 0.91 to 0.96.

Hot band with a thickness of -17.5 mm was further processed to an intermediate band by rolling at 550°C an additional 20% and 40%. Charpy V-notch specimens were cut from the hot band before and after cold rolling by wire EDM and tested according to the procedures described in the Main Body of the current application. The results are listed in Table 12. Absorbed energy values during Charpy V-notch testing of alloys herein are in a range from 89 to 171 J with Directional Toughness Ratio (DTR) from 0.94 to 1.20.

Table 12 Charpy V-Notch Energy In Alloy 88 After Different Rolling

This Case Example demonstrates that rolling of hot band from alloys herein at ambient and intermediate temperatures maintain toughness uniformity with a Directional Toughness Ratio (DTR) from 0.91 to 1.20.

Case Example #6 Tensile Properties After Hot Rolling to High Thickness

The Alloy 66 was processed into a laboratory hot band by hot rolling of laboratory cast 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 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 varied between 1100°C to 1250°C, depending on alloy melting point and the 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 two-high rolling mill. The 50 mm casts were hot rolled for 5 and 10 passes though the mill before being allowed to air cool. Final thickness after hot rolling was 11.9 and 17.0 mm.

Tensile specimens were cut from laboratory hot band using wire EDM. Since laboratory tensile testing machine is not capable of testing samples with >10 mm thickness, the hot band thick specimens were the “sliced” into multiple specimens with -1.6 mm thickness. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron’s Bluehill control and analysis software. Tensile testing results are listed in Table 13 representing the properties of the thick hot band. The ultimate tensile strength values of the thick hot band are in a range from 914 to 1060 MPa, the yield strength at 0.2% offset varies from 276 to 340 MPa, the total elongation recorded in the range from 30.4 to 46.4%, and area under tensile stress-strain curve is in a range from 20,149 to 35,535 MPa%. Strength ductility product is calculated as UTS multiplied by total tensile elongation varies from 27,959 to 49,174 MPa%. A Tensile Squareness Ratio (TSR) is determined as the area under stress - strain curve up to UTS divided by the product of UTS multiplied by strain at UTS (FIG. 4) and the calculated values are in a range from 0.71 to 0.73. Note that the Table 13 properties correspond to Step 2 in FIG. 1 and FIG. 2.

Table 13 Tensile Properties Of Thick Hot Band From Alloy 66

This Case Example demonstrates tensile properties of the hot band from alloys here with thickness greater than 10 mm with a Tensile Squareness Ratio (TSR) from 0.71 to 0.73.

Case Example #7 Effect of Rolling at Ambient Temperature on Yield Strength of the Hot Band

Slabs of Alloy 66, Alloy 80 and Alloy 84 were cast according to the elemental composition provided in Table 1 at 80 mm thickness. The slabs were heated to 1250°C for 40 minutes then hot rolled to 18 mm thickness over seven passes. The slabs were reheated to 1100°C and then rolled to 12 mm thickness in one pass and allowed to cool to room temperature. Due to the high thickness of the hot band, the thickness was sliced to approximately 1.6 mm thick for tensile property testing. Ten tensile specimens were cut from the hot band by wire EDM. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron’s Bluehill control and analysis software.

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. Tensile testing results are listed in Table 14 (hot rolled condition). The yield strength at 0.2% offset varies from 307 to 320 MPa and a Tensile Squareness Ratio (TSR) is determined in a range from 0.72 to 0.82. The area under the stress-strain curve corresponding to the work require to strain the material to 10% (Slo_.i) is in a range from 4,936 to 8,446 MPa%, to strain to 20% (SI 0_.2) is in a range from 11,618 to 18,193 MPa%, and to strain to 30% (SI 0_.3) is in a range from 20,015 to 26,899 MPa%.

The hot band from each alloy was rolled at ambient temperature with reductions between 2 and 9%. Note that while no external heating was applied, there was some temperature rise depending on the amount of the reduction and the time between passes, so the temperature range was from 15 to up to 50°C. Due to the high thickness of the cold rolled material, the thickness was sliced to approximately 1.6 mm thick for tensile property testing. Between ten and twelve tensile specimens were cut from each cold rolled plate by wire EDM and tested according to the procedures described above. The average values of the measured 0.2% offset yield strength and Tensile Squareness Ratio in each condition is provided in Table 14. Both yield strength and Tensile Squareness Ratio increase with increasing cold rolling reduction. The average yield strength (Y2) is in a range from 403 to 562 MPa and Y2>Y1. The average Tensile Squareness Ratio (TSR2) is between 0.78 and 0.92 and TSR2>TSR1. The area under the stress-strain curve corresponding to the work require to strain the material to 10% (S2o_.i) is in a range from 5,918 to 8,510 MPa%, to strain to 20% (S2o_.2) is in a range from 13,159 to 18,721 MPa%, and to strain to 30% (S2O_.3) is in a range from 21,310 to 30,015 MPa%.

Yield strength (Y2) as a function of rolling reduction at ambient temperature is demonstrated in FIG. 22, FIG. 23 and FIG. 24 for Alloy 66, Alloy 80 and Alloy 84, respectively. Tensile Squareness Ratio (TSR2) as a function of rolling reduction is shown in FIG. 25, FIG. 26 and FIG. 27 for Alloy 66, Alloy 80, and Alloy 84, respectively. Note that this data in the Table 14 correspond to Step 3 in FIG. 2.

Table 14 Yield Strength As A Function Of Rolling Reduction At Ambient Temperature

This Case Example demonstrates that the yield strength of the hot band from alloys herein increases as a function of rolling reduction at ambient temperature forming alloys with yield strength Y2>Y1 and Tensile Squareness Ratio TSR2>TSR1, where Y1 and TSR1 are yield strength and Tensile Squareness Ratio in a hot band before rolling.

Case Example #8 Effect of Rolling at Intermediate Temperature on Yield Strength of the Hot Band

Slabs of Alloy 66, Alloy 80 and Alloy 84 were cast according to the elemental composition provided in Table 1 at 80 mm thickness. The slabs were heated to 1250°C for 40 minutes then hot rolled to 25 mm thickness over six passes. The slabs were reheated to 1100°C and then rolled to 18 mm thickness in one pass and allowed to cool to room temperature. Due to the high thickness of the hot band, the thickness was sliced to approximately 1.6 mm thick for tensile property testing. Sixteen reduced geometry tensile specimens were cut from the hot band by wire EDM. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron’s Bluehill control and analysis software.

Hot band material from Alloy 66 was heated to 550°C for 40 minutes, then rolled at to approximately 10%, 20%, 30%, and 40% reductions. The thickness was sliced to approximately 1.6 mm thick for tensile property testing. Between ten and twelve reduced geometry tensile specimens were cut for each rolled condition by wire EDM and tested according to the procedures described above. Hot band material from Alloy 80 was heated to 600°C for 40 minutes, then rolled to approximately 10%, 20%, 30%, and 40% reductions. The thickness was sliced to approximately 1.6 mm thick for tensile property testing. Between ten and twelve reduced geometry tensile specimens were cut for each rolled condition by wire EDM and tested according to the procedures described above. Hot band material from Alloy 84 was rolled with approximately 10% reduction at four intermediate temperatures (50, 150, 250 and 350°C). The thickness was sliced to approximately 1.6 mm thick for tensile property testing. Between ten and twelve reduced geometry tensile specimens were cut for each condition by wire EDM and tested according to the procedures described above. The average values of the measured 0.2% offset yield strength and Tensile Squareness Ratio in each condition are provided in Table 15 and Table 16.

The yield strength at 0.2% offset in hot rolled condition (Yl) varies from 319 to 331 MPa and a Tensile Squareness Ratio (TSR) is determined in a range from 0.70 to 0.83. The area under the stress- strain curve corresponding to the work require to strain the material to 10% (Slo_.i) is in a range from 4,882 to 4,998 MPa%, to strain to 20% (SI0_.2) is in a range from 11,518 to 11,701 MPa%, and to strain to 30% (SI 0_.3) is in a range from 19,194 to 19,831 MPa%.

Both yield strength and Tensile Squareness Ratio increase with increasing rolling reduction. The average yield strength (Y2) is in a range from 393 to 746 MPa and Y2>Y1. The average Tensile Squareness Ratio (TSR2) is between 0.78 and 0.91 and TSR2>TSR1. The area under the stress-strain curve corresponding to the work require to strain the material to 10% (S2o_.i) is in a range from 6,466 to 10,007 MPa%, to strain to 20% (S2o_.2) is in a range from 14,754 to 20,762 MPa%, and to strain to 30% (S2o_.3) is in a range from 23,962 to 32,102 MPa%. Note that this data in the Table 15 correspond to Step 3 in FIG. 2. Table 15 Yield Strength As A Function Of Rolling Reduction Table 16 Tensile Properties As A Function Of Rolling Temperature

Yield strength (Y2) as a function of rolling reduction is demonstrated in FIG. 28 and FIG. 29 for Alloy 66 and Alloy 80, respectively. Yield strength (Y2) as a function of rolling temperature is shown in FIG. 30 for Alloy 84. Tensile Squareness Ratio (TSR2) as a function of rolling reduction is shown in FIG. 31 and FIG. 32 for Alloy 66 and Alloy 80, respectively. Tensile Squareness Ratio (TSR2) as a function of rolling temperature is demonstrated in FIG. 33 for Alloy 84.

This Case Example demonstrates that the yield strength of the hot band from alloys herein increases as a function of a reduction during rolling at intermediate temperatures forming alloys with yield strength Y2>Y1 and Tensile Squareness Ratio TSR2>TSR1, where Y1 and TSR1 are yield strength and Tensile Squareness Ratio in hot band before rolling.

Case Example # 9 Rolling Effect on Alloy 88 Hot Band Properties

Laboratory slab with thickness of 80 mm was cast from Alloy 88 according to the atomic ratios provided in Table 1 and laboratory processed by hot rolling as described in the Main Body section of the current application. The thickness was also sliced to approximately 1.6 mm thick for tensile property testing. Tensile specimens were cut for each rolled condition by wire EDM and tested according to the procedures described above. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron’s Bluehill control and analysis software. Tensile properties are listed in Table 17. In hot rolled condition, a yield strength at 0.2% offset (Y2) varies from 268 to 319 MPa and a Tensile Squareness Ratio (TSR2), determined as the area under stress - strain curve up to UTS divided by the product of UTS multiplied by strain at UTS (FIG. 4), is in range from 0.77 to 0.82. The area under the stress-strain curve corresponding to the work require to strain the material to 10% (Slo_.i) is in a range from 4,835 to 4,969 MPa%, to strain to 20% (SI0_.2) is in a range from 11,499 to 11,787 MPa%, and to strain to 30% (SI0_.3) is in a range from 19,439 to 19,880 MPa%.

Hot band with a final hot rolled thickness of -11.9 mm was further rolled at ambient temperature with additional 3% and 9% reduction. The thickness was also sliced to approximately 1.6 mm thick for tensile property testing. Tensile specimens were cut for each rolled condition by wire EDM and tested according to the procedures described above. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron’s Bluehill control and analysis software. Tensile properties are listed in Table 17. In the material after rolling at ambient temperature, a yield strength at 0.2% offset (Y2) varies from 327 to 801 MPa and a Tensile Squareness Ratio (TSR2), determined as the area under stress - strain curve up to UTS divided by the product of UTS multiplied by strain at UTS (FIG. 4), is in range from 0.82 to 0.93. The area under the stress-strain curve corresponding to the work require to strain the material to 10% (S2o_.i) is in a range from 6,068 to 9,349 MPa%, to strain to 20% (S2o_.2) is in a range from 13,891 to 19,555 MPa%, and to strain to 30% (S2o_.3) is in a range from 22,782 to 30,305 MPa%.

Hot band with a thickness of -17.5 mm was further processed to an intermediate band by rolling at 550°C an additional 20% and 40%. The thickness was also sliced to approximately 1.6 mm thick for tensile property testing. Tensile specimens were cut for each rolled condition by wire EDM and tested according to the procedures described above. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron’s Bluehill control and analysis software. Tensile properties are listed in Table 17. In the material after rolling at intermediate temperature, a yield strength at 0.2% offset (Y2) varies from 529 to 840 MPa and a Tensile Squareness Ratio (TSR2), determined as the area under stress - strain curve up to UTS divided by the product of UTS multiplied by strain at UTS (FIG. 4), is in range from 0.89 to 0.94. The area under the stress- strain curve corresponding to the work require to strain the material to 10% (S2o_.i) is in a range from 7,624 to 8,174 MPa%, to strain to 20% (S2o_.2) is in a range from 16,575 to 17,647 MPa%, and to strain to 30% (S2o_.3) is in a range from 26,179 to 27,767 MPa%.

Table 17 Properties Of Alloy 88 After Different Rolling

Standard V-notch Charpy specimens (FIG. 6) were cut by wire EDM for instrumented Charpy impact testing of the hot band before and after rolling at ambient and intermediate temperatures. The Charpy machine used is instrumented with a small force sensor to record load and an encoder to record hammer velocity. The results are listed in Table 18. Based on material response to impact, its fracture behavior can be represented by four types with Type IV being most ductile (FIG. 17). Force-displacement data were captured in addition to the total energy absorbed and corresponding curves are shown in FIG. 34 through FIG. 39. A V-notch Charpy impact energy of the material in the hot rolled condition is measured from 293 to 319 J and after rolling at ambient or intermediate temperatures it varies from 105 to 160 J. A dynamic fracture toughness (J0_.2) was calculated based on test results and it is listed in Table 18. The fracture toughness values reported are for notched Charpy specimens. The final fracture toughness values are reported with the subscript “0.2” to indicate the final recommended fracture toughness is the value of Ji_dn at a crack extension of 0.2 mm. The “Idn” subscript indicates Mode I loading, dynamic testing and a notch was used to initiate the crack. The fracture toughness values do not meet the El 820 size requirement Ji_dn<J_max=Boo/10 and should be considered an estimate. Note that this data in the Table 17 and Table 18 correspond to FIG. 2.

Table 18 Instrumented Charpy Test Data For Alloy 88

This Case Example demonstrates that a hot band from Alloy 88 exhibits a ductile Type IV behavior in initial state as well as after rolling at ambient or intermediate temperatures. Both rolling at ambient temperature and rolling at intermediate temperature form alloys with yield strength, Y2>Y1, and a Tensile Squareness Ratio, TSR2>TSR1, (TSR) that and Tensile Squareness Ratio, where Y 1 and TSR1 are yield strength and Tensile Squareness Ratio in hot band before rolling.

Finally, it is worth noting herein that another application of the alloy herein, where relatively uniform or isotropic toughness and combinations of relatively high yield and ultimate tensile strength are important, are for battery protection in plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs). Both of these vehicle types share the commonality that they utilize batteries / battery packs to store energy for subsequent propulsion. Protecting the batteries from all conceivable stresses and external impacts is one key application where the alloy sheet produced in Step (2) of Fig. 1 or Step 3 of Fig. 2 would be utilized. There are a myriad of potential designs for the exo-skeleton, battery tray, or battery cage to protect the batteries from impingement, penetration, and damage from an external contact or crash event.

Claims

1. A method to achieve a combination of properties including toughness and a directional toughness ratio (DTR) in hot band from high strength steel alloys involving: a. supplying a metal alloy comprising at least 65 at.% Fe with Mn, Cr, Si, and C, and optionally Ni and/or Cu; melting said alloy, cooling at a rate of < 250 K/s, and solidifying to a thickness of 25.0 mm up to 500 mm; b. processing said alloy by heating and reducing said thickness by rolling the metal alloy in step (a) in a selected direction to form a sheet with thickness of 10.0 to 20.0 mm, optionally subjecting said alloy sheet to a temperature from 600°C up to but not including T_m, where T_m is a melting point of said alloy, to produce said alloy sheet with a total elongation, El from 30 to 75%, a yield strength at 0.2% offset, Yl, from 250 to 525 MPa, an ultimate tensile strength, Ul, from 750 to 1400 MPa and a Tensile Squareness Ratio, TSR1, from 0.65 to 0.90, wherein:

2. The method of claim 1 wherein the alloy in a) comprises Fe, Mn, Cr, Si and C, and additionally at least one element selected from Ni and Cu.

3. The method of claim 1 wherein the alloy in a) contains 65 to 80 at% Fe, 9.5 to 17.5 at.% Mn, 1.0 to 10.0 at.% Cr, 1.0 to 5.5 at.% Si, and 0.5 to 1.5 at.% C and if selected, contains 0.2 to 4.0 at.% Ni, and/or 0.1 to 2.5 at.% Cu.

4. The method of claim 1 wherein the alloy in (a) indicates a solidus temperature from 1350°C to 1450°C, a liquidus temperature from 1400°C to 1500°C, and a liquidus to solidus gap from 40°C to 100°C.

5. The method of claim 1 wherein the alloy sheet in (b) has a density from 7.7 g/cm³ to 8.0 g/cm³.

6. The method of claim 1 wherein the alloy sheet in b) exhibits an area under tensile curve S lo_.i in a range from 4,500 to 8,500 MPa%, Slo.2 in a range from 10,500 to 18,500 MPa%, and SI 0.3 in a range from 17,500 to 27,000 MPa%.

7. The method of claim 1 wherein the alloy sheet in b) exhibits a strength / elongation product from 25,000 to 80,000 MPa%.

8. The method of claim 1 wherein the alloy sheet in b) exhibits an area under tensile curve from 20,000 to 65,000 MPa%.

9. The method of claim 1 wherein the alloy sheet in b) is impacted both in the normal and in the transverse orientations;

10. The method of claim 1 wherein the impacted alloy sheet in b) exhibits a Charpy V-notch toughness J1 from 150 J to 850 J.

11. The method of claim 1 wherein said alloy sheet in step b) is positioned as all or part of a storage tank, freight car, railway tank car.

12. The method of claim 1 wherein said alloy sheet in step b) is positioned as all or part of a storage tank, freight car, railway tank car and the impact energy absorbed occurs on the positioned alloy sheet.

13. The method of claim 1 wherein said alloy sheet formed in step b) is positioned as all or part of a vehicular frame, vehicular chassis, vehicular panel, battery exo-skeleton, battery tray, or battery cage.

14. The method of claim 1 wherein said alloy sheet formed in step b) is positioned as all or part of a vehicular frame, vehicular chassis, vehicular panel, battery exo-skeleton, battery tray, or battery cage and the impact energy absorbed occurs on the positioned alloy sheet.

15. The method of claim 1 wherein the alloy sheet in c) exhibits a microvoid coalescence mechanism during fracture.

16. A method to achieve novel combinations of properties including yield strength and a tensile squareness ratio (TSR) in hot band from high strength steel alloys involving: a. supplying a metal alloy comprising at least 65 at.% Fe with Mn, Cr, Si, and C, and optionally Ni and/or Cu; melting said alloy, cooling at a rate of < 250 K/s, and solidifying to a thickness of 25.0 mm up to 500 mm; b. processing said alloy by heating and reducing said thickness to form a sheet with thickness of 10.0 mm to 20.0 mm, optionally subjecting said alloy sheet to a temperature from 600°C up to but not including T_m, where T_m is a melting point of said alloy, to produce said alloy sheet with a total elongation, El from 30 to 75%, a yield strength at 0.2% offset, Yl, from 250 to 525 MPa, an ultimate tensile strength, Ul, from 750 to 1400 MPa and a Tensile Squareness Ratio, TSR1, from 0.65 to 0.90; c. subjecting said alloy sheet to a rolling reduction in thickness at: (1) a first temperature range T1 of 15 °C to < 50 °C with a reduction in thickness of said sheet in step (b) of 1 to 10%; or (2) at a second temperature range of 50 °C to < 600 °C with a reduction in thickness of said sheet in step (b) of 10% to 40% to produce an alloy sheet having a yield strength Y2>Y1, and Tensile Squareness Ratio TSR2>TSR1.

17. The method of claim 16 wherein the alloy in a) comprises Fe, Mn, Cr, Si and C, and additionally at least one element selected from Ni and Cu.

18. The method of claim 16 wherein the alloy in a) contains 65 to 80 at% Fe, 9.5 to 17.5 at.% Mn, 1.0 to 10.0 at.% Cr, 1.0 to 5.5 at.% Si, and 0.5 to 1.5 at.% C and if selected, contains 0.2 to 4.0 at.% Ni, and/or 0.1 to 2.5 at.% Cu.

19. The method of claim 16 wherein the alloy in (a) indicates a solidus temperature from 1350°C to 1450°C, a liquidus temperature from 1400°C to 1500°C, and a liquidus to solidus gap from 40°C to 100°C.

20. The method of claim 16 wherein the alloy sheet in (b) has a density from 7.7 g/cm³ to 8.0 g/cm³.

21. The method of claim 16 wherein the alloy sheet in b) exhibits an area under tensile curve Slo_.i in a range from 4,500 to 8,500 MPa%, Slo.2 in a range from 10,500 to 18,500 MPa%, and SI 0.3 in a

22. The method of claim 16 wherein the alloy sheet in c) exhibits a yield strength Y2 > Yl.

23. The method of claim 16 wherein the alloy sheet in c) exhibits a Tensile Squareness Ratio TSR2 > TSR1.

24. The method of claim 16 wherein the alloy sheet in c) exhibits a yield strength, Y2, from 300 to 850 MPa.

25. The method of claim 16 wherein the alloy sheet in c) exhibits a Tensile Squareness Ratio, TSR2, from 0.75 to 0.95.

26. The method of claim 16 wherein said alloy sheet in step c) is positioned as all or part of a storage tank, freight car, railway tank car.

27. The method of claim 16 wherein said alloy sheet formed in step c) is positioned in a vehicular frame, vehicular chassis, vehicular panel, battery exo-skeleton, battery tray, or battery cage.