Patent Publication Number: US-2022219423-A1

Title: Lightweight steel

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 16/841,549, filed Apr. 6, 2020, which claims priority to U.S. Provisional Patent Application No. 62/829,236, filed Apr. 4, 2019, the entire contents of both of which are incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to lightweight steel and a method for manufacturing lightweight steel. 
     BACKGROUND 
     Metal is considered a foam if pores or voids are distributed within the metal to take up a certain minimum percentage of the total volume of the metal. The introduction of pores or voids into a metal component typically decreases the density and weight of the metal component compared to a solid metal component. Metal foam components also frequently display a higher plate bending stiffness and other desirable mechanical properties than solid metal components. Currently, commercial metal foam components are generally limited to aluminum, despite the fact that steel foam components would exhibit many superior properties if they could be produced in volume at reasonable cost. 
     Hot rolling is a metal forming process in which a metal is passed through one or more pairs of rolls to reduce the thickness of the metal and make the thickness throughout the metal uniform. The temperature of the metal being rolled is typically above the recrystallization temperature of the metal. 
     SUMMARY 
     Embodiments of the present invention provide the ability to produce lightweight steel plates having consistent densities. In addition, embodiments of the present invention provide the ability to produce steel plates having predictable and enhanced mechanical properties. 
     Aspects of the present invention provide engineers working with steel a new degree of freedom: density. The design space potentially covered by steel applications can grow significantly with density as a variable. Among other things, the present invention opens new opportunities for designers to find suitable military and naval applications for not only energy absorption, but also blast resistant and ballistic applications to resist the impact of objects due to the high strength and hardness of products produced in accordance with structures and methods presented herein. 
     In some embodiments, the present invention provides a lightweight steel slab including a first panel defining a first plane, a second panel spaced apart from the first panel and defining a second plane, and a truss structure coupled to the first panel and the second panel. The truss structure includes a plurality of struts extending between the first panel and the second panel. At least some of the plurality of struts includes linear elements that are obliquely angled relative to the first plane and the second plane. 
     In some embodiments, the present invention provides a lightweight steel slab including a first panel having a first end, a second end, and a length extending between the first end and the second end, a second panel spaced apart from the first panel, and a truss structure integrally formed as a single piece with the first panel and the second panel. The truss structure includes a plurality of struts extending between the first panel and the second panel. The plurality of struts is distributed consistently and repeatedly along the length of the first panel. 
     In some embodiments, the present invention provides a lightweight steel slab including a first panel, a second panel spaced apart from the first panel, and an auxetic truss structure coupled to the first panel and the second panel. The auxetic truss structure includes a plurality of struts extending between the first panel and the second panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a lightweight steel slab according to an embodiment of the invention. 
         FIG. 2  is a perspective view of another lightweight steel slab according to an embodiment of the invention. 
         FIG. 3  is a cross-sectional perspective view of the lightweight steel slab of  FIG. 2  taken along section line  3 - 3 . 
         FIG. 4  is a perspective view of another lightweight steel slab according to an embodiment of the invention. 
         FIG. 5  is a cross-sectional perspective view of the lightweight steel slab of  FIG. 4  taken along section line  5 - 5 . 
         FIG. 6  is a cross-sectional perspective view of the lightweight steel slab of  FIG. 4  taken along section line  6 - 6 . 
         FIG. 7  is a perspective view of another lightweight steel slab according to an embodiment of the invention. 
         FIG. 8  is a cross-sectional view of the lightweight steel slab of  FIG. 7  taken along the section line  8 - 8 . 
         FIG. 9  is a perspective view of another lightweight steel slab according to an embodiment of the invention. 
         FIG. 10  is a cross-sectional view of the lightweight steel slab of  FIG. 9  taken along the section line  9 - 9 . 
         FIG. 11  is a perspective view of another lightweight steel slab according to an embodiment of the invention. 
         FIG. 12  is a cross-sectional view of the lightweight steel slab of  FIG. 11  taken along the section line  12 - 12 . 
         FIG. 13  is a perspective view of another lightweight steel slab according to an embodiment of the invention. 
         FIG. 14  is a cross-sectional view of the lightweight steel slab of  FIG. 13  taken along the section line  14 - 14 . 
         FIG. 15  is a perspective view of another lightweight steel slab according to an embodiment of the invention. 
         FIG. 16  is a cross-sectional view of the lightweight steel slab of  FIG. 15  along the section line  16 - 16 . 
         FIG. 17  is a perspective view of a lightweight steel sheet produced through hot rolling the lightweight steel slab of  FIG. 1 . 
         FIG. 18  is a perspective view of a lightweight steel sheet produced through hot rolling the lightweight steel slab of  FIG. 2 . 
         FIG. 19  is a perspective view of a lightweight steel sheet produced through hot rolling the lightweight steel slab of  FIG. 4 . 
         FIG. 20  is a flow chart depicting a method of producing lightweight steel sheets from lightweight steel slabs through hot rolling. 
         FIG. 21  is a perspective view of an insert used to form the lightweight steel slabs of  FIGS. 2-6  according to an embodiment of the invention. 
         FIG. 22  is a perspective view of an insert used to form the lightweight steel slabs of  FIGS. 7 and 8  according to an embodiment of the invention. 
         FIG. 23  is a perspective view of an insert used to form the lightweight steel slabs of  FIGS. 9 and 10  according to an embodiment of the invention. 
         FIG. 24  is a perspective view of an insert used to form the lightweight steel slabs of  FIGS. 11 and 12  according to an embodiment of the invention. 
         FIG. 25  is a perspective view of an insert used to form the lightweight steel slabs of  FIGS. 13 and 14  according to an embodiment of the invention. 
         FIG. 26  is a perspective view of an insert used to form the lightweight steel slabs of  FIGS. 15 and 16  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before embodiments of the present invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     Some embodiments of steel slabs according to the present invention are described in U.S. Pat. No. 9,623,480 filed Dec. 19, 2014 and U.S. Pat. No. 10,493,522 filed Jun. 2, 2017, both of which are entitled “STEEL FOAM AND METHOD FOR MANUFACTURING STEEL FOAM,” the entire contents of both of which are hereby incorporated by reference. Embodiment of steel slabs according to the present invention have any combination of the chemical component elements set forth below. The component elements presented below can include limitations for the reasons described below, where the unit “%” relating to the chemical component elements in the steel refers to “mass %” unless specified otherwise. 
     C: 0.1% to 0.35% 
     Carbon (C) is an element used in steels to achieve surface hardness and strength, which is determined by the percentage of carbon and subsequent heat treatment. Generally, low carbon content steels provide improved toughness. Higher carbon contents provide higher strength, hardness, and hardenability. Carbon in excess of 0.35% contributes to increased brittleness and reduced weldability. Therefore, it is preferably that the carbon content is in the range of 0.10% to 0.35%. 
     Si: 0.2% to 0.8% 
     Silicon (Si) has high work hardenability to ensure that ductility is not substantially decreased as the strength is increased, thereby contributing to providing an improved balance between strength and ductility after heat treatment. In addition, silicon is an element typically required to improve material homogeneity by promoting ferrite transformation in the hot rolling stage, and securing a desired grain size and a desired volume fraction. Silicon content of greater than 0.2% produces such an effect. If silicon content exceeds 0.8%, hot-dip galvanizing properties after annealing deteriorate significantly. 
     Mn: 0.5% to 1.5% 
     Manganese (Mn) is an element that can contribute to the hardenability of steels, meaning increasing the depth of hardness. In some embodiments, the Mn content is less than 1.5% due to the tendency of Mn to segregate, and the adverse effect of Mn on quench cracking. In quantities greater than 0.5%, Mn can reduce the potential for harmful Type II MnS inclusions in steel. The desired ratio of Mn to Sulfur is 10 to 1. Therefore, the desirable range of Mn is 0.5% to 1.5% 
     P: less than 0.025% 
     Phosphorous (P) is an element that can contribute to an increase in the ductile-to-brittle transition temperature, and reduced toughness and ductility in steels. Phosphorous is generally an undesirable element in steels, which preferably is limited to a phosphorous content of less than 0.025%. 
     S: less than 0.025% 
     Sulfur (S) in high quantities may form low-melting iron sulfide, a grain boundary phase that causes severe hot shortness during hot rolling if manganese content is not sufficient to counter this effect. Sulfur can also decrease toughness and ductility in steels. Sulfur is generally an undesirable element in steels, which preferably is limited to a sulfur content of less than 0.025%. 
     Al: 0.01% to 0.08% 
     Aluminum (Al) is an element often required for deoxidation, which also prevents the formation of gas porosity. Al is also useful in preventing grain growth during heat treatment. In some embodiments, the content of Al is greater than 0.01% to produce such an effect. However, since Al content exceeding 0.08% can lead to the formation of aluminum nitride phase at the grain boundaries and can reduce the toughness and ductility of steel, Al content of 0.08% or less is preferred. 
     Ni: less than 2.00% 
     Nickel (Ni) is an element contributing to high strengthening of steel by reducing the ductile-to-brittle transition temperature in steel, and also contributing to high strengthening by increasing quench hardenability during heat treatment. Nickel content of greater than 2.00% can contribute to steels that are prone to the formation of undesirably retained austenite during heat treatment, since nickel is an austenite stabilizer. Therefore, it is preferred that the content of Nickel is less than 2.00%. 
     Cr: less than 1.5% 
     Chromium (Cr) is an element contributing to high strengthening of steel by improving hardenability during heat treatment. If Cr content exceeds 1.5%, quench cracking in low alloy steels typically increases. Cr is ordinarily used with other alloying elements, such as Molybdenum. Thus, Cr content of less than 1.5% is preferred. 
     Mo: less than 0.5% 
     Molybdenum (Mo) is an element contributing to high strengthening of steel by increasing quench hardenability, and is typically present in high strength low alloy steels. Mo typically reduces the temper embrittlement in steels, and improves the toughness of low alloy steels. If Mo content exceeds 0.5%, no improvement in the effect is recognized in some embodiments. Thus, Mo content of less than 0.5% is preferred. 
     In the chemical compositions as explained above, the balance of the steel disclosed herein is iron (Fe), and can also include incidental impurities. 
     The incidental impurities can include, for instance, Sb, Sn, Zn, and Co, and their permissible ranges can be Sb: 0.01% or less; Sn: 0.1% or less; Zn: 0.01% or less; and Co: 0.1% or less. In addition, Ti and Zr may be contained within ranges of ordinary steel compositions, to the extent that the desired effects are not lost. In further embodiments, steel slabs according to the present invention may include chemical compositions that differ from the chemical compositions detailed above. For example, the chemical composition may be that of high alloy steels such as stainless steels, or the like. 
       FIG. 1  illustrates a steel slab  100  according to an embodiment of the present invention, prior to a hot rolling cycle. The illustrated steel slab  100  includes a body  104  in the shape of a rectangular prism. The body  104  includes a first face  108  that is generally square in shape, a second face  112  that is generally square in shape and located opposite the first face, and a peripheral edge  116  extending between the first face  108  and the second face  112 . The first face  108  and the second face  112  each define an outside face of a first panel  117 . In other embodiments, the body  104  may be other desired shapes, and the first face  108  and the second face  112  may likewise have different shapes. As shown, the peripheral edge  116  is four-sided, although the peripheral edge  116  can have fewer or more sides in other embodiments. A distance between the first face  108  and the second face  112  represents a thickness of the peripheral edge  116 , which in some embodiments is the smallest dimension of the body  104  in comparison to larger length and width dimensions. In some embodiments, the thickness is greater than 1 inch. In other embodiments, the thickness is less than 2 inches. In the illustrated embodiment, the thickness is within a range of 1 inch to 2 inches. In further embodiments, the thickness may be less than 1 inch or greater than 2 inches. 
     With continued reference to the illustrated embodiment of  FIG. 1 , the body  104  also includes a plurality of interconnected pores  120 . In the illustrated embodiment, the interconnected pores  120  form a substantially uniform pattern within the steel slab  100 . The substantially uniform pattern can be a generally two-dimensional pattern (a grid, array, or lattice, by way of example only) that can define a rectangular pattern within the body  104 . In other embodiments, the substantially uniform pattern can be a three-dimensional pattern that can also define a rectangular or prismatic pattern within the body  104 . In other embodiments, the two-dimensional or three-dimensional arrangement of pores  120  do not define an identifiable pattern as just described, yet still define a network of interconnected pores  120  within the body  104 . The pores  120  are empty voids in the steel slab  100 , with each of the plurality of pores  120  connected to at least one other of the plurality of pores  120 . In some embodiments, the pores  120  are spherical in shape, and are connected together by cylindrically-shaped voids. In other embodiments, the pores  120  may have any other desired or suitable shape, such cubes, pyramids, or other prisms, cones, tetrahedrons, octahedrons, dodecahedrons, icosahedrons, ellipsoids, tori, ovular shapes, irregular faceted and non-faceted shapes, and the like. 
     Although in some embodiments (such as in the illustrated embodiment of  FIG. 1 ) the pores  120  have the same shape, in other embodiments two or more pore shapes exist within the body  104 . In such embodiments, pores  120  having different shapes can be interconnected as described above, or different unconnected networks of pores can each have a common pore shape that differs from pore network to pore network. Similarly, although in some embodiments (such as the illustrated embodiment of  FIG. 1 ) the pores have the same size, in other embodiments two or more pore sizes exist within the body  104 . In such embodiments, pores  120  having different sizes can be interconnected as described above, or different unconnected networks of pores can each have a common pore size that differs from pore network to pore network. 
     The pores  120  in the illustrated embodiment are arranged in a series of pore rows  124  and pore columns  128 , with the pore rows  124  being parallel to a horizontal axis H, and the pore columns  128  being parallel to a vertical axis V. As used herein, terms of orientation such as “horizontal” and “vertical” are for ease of description only, and are not intended to be limiting absent specific reference to the same in the appended claims. As also shown in the embodiment of  FIG. 1 , pores  120  of the body  104  communicate through the peripheral edge  132  of the lightweight steel component  100 . Any number of pores  120  can be open to any one or more of the first and second faces  108 ,  112 , the peripheral edge  116 , and/or other exterior surfaces of the body  104  in other embodiments. 
     In the illustrated embodiment, the plurality of pores  120  forms at least 20% of a total volume of the steel body  104 . In some embodiments, the pores  120  may form 20% of the total volume. In other embodiments, the pores  120  may form at least 50% of a total volume of the steel body  104 . In other embodiments, the pores  120  may form between 20% and 50% of the total volume. In further embodiments, the pores  120  may form less than 50% of the total volume. In still other embodiments, the pores  120  may form 25%, 30%, 35%, 40%, or 45% of the total volume of the steel body  104 . In some embodiments, the calculation of volume (in which the plurality of pores  120  occupies) is made by identifying the bounds of the volume in the body  104  in which the pores  120  exist, and then calculating the space occupied within that volume. Such a calculation can be made, for example, in products in which only a portion of the body  104  has pores  120 . 
     As just described, in some embodiments the plurality of interconnected pores  120  does not extend through an entirety of the steel body  104 , such that at least a portion of the steel body  104  is solid. For example, one or more sections of the steel body  104  may include pores to decrease the density of that section(s), while one or more other sections of the steel body  104  may not include pores  120 , and may be entirely or relatively solid. By way of example only, the one or more sections with pores  120  may be located in a center of the steel body  104 , near one or more edges of the steel body  104 , or both. In these and other embodiments, the pores  120  may create a gradient density within the steel body  104  such that the density of pores  120  is different in different locations in the body  104 . 
       FIGS. 2 and 3  illustrate a steel slab  200  prior to the hot rolling cycle according to another embodiment of the present invention. The steel slab  200  includes a body  204  having pores  220  arranged in a series of pore rows  224  and pore columns  228 , similar to the steel slab  100  of  FIG. 1 . The pores  220  of  FIGS. 2 and 3 , however, are empty voids shaped as triangular prisms. The triangular prism voids in the illustrated embodiment are disposed at alternating pore rows  224 . In other words, the triangular prism voids are disposed at every other pore row  224 . At the pore rows  224  that include the triangular prism voids, the triangular prism voids are equally spaced along the pore row  224  such that every pore column  228  includes the triangular prism voids. At the pore rows  224  that do not include triangular prism voids, a rectangular prism void is formed. The rectangular prism voids extend from one peripheral edge  216 , to an opposite peripheral edge  216  of the body  204  such that the pore row  224  forms a continuous void. The rectangular prism voids allow communication between triangular prism voids at different pore rows  224  along the pore columns  228 . In other words, the rectangular prism voids interconnect the triangular prism voids to form the interconnected pores  220 . 
     With continued reference to the illustrated embodiment of  FIGS. 2 and 3 , the illustrated pores  220  form a truss structure or strut structure  230  between a first panel  217  and a second panel  218  of the steel slab  200 . The strut structure  230  is defined by a plurality of struts  232  extending between the first panel  217  and the second panel  218 . Each strut  232  in the illustrated embodiment is a linear element, although in other embodiments some or all of the struts  232  can have other shapes connecting the first and second panels  217 ,  218 , such as curved, stepped, and/or tapered struts. Each strut  232  in the illustrated embodiment is also obliquely angled relative to planes defined by the first panel  217  and the second panel  218 . Also in the illustrated embodiment, each strut  232  is angled to a similar degree. For example, each strut  232  may be angled approximately 60 degrees relative to the planes defined by the first panel  217  and the second panel  218 . Each strut  232  may alternatively be angled at different degrees, such as 45 degrees, 30 degrees, and the like. In other embodiments, the struts  232  may be oriented at still other angles relative to each other, and/or can each be oriented with respect to the first and second panels  217 ,  218  at different respective oblique angles. The illustrated struts form truss-like structure between the first panel  217  and the second panel  218 . 
     The first panel  217  and second panel  218  of the illustrated embodiment are flat panels. The panels  217 ,  218  may also be referred to as skins. Each panel  217 ,  218  is generally rectangular and planar. In the illustrated embodiment, the first panel  217  and the second panel  218  have similar thicknesses. In other embodiments, the panels  217 ,  218  may have unequal thicknesses. Additionally or alternatively, the first and second panels  217 ,  218  may have other (e.g., non-planar and/or non-rectangular) shapes, such as circular, oblong, and the like, or any of the other shapes described above in connection with the body  104  illustrated in  FIG. 1 . Further, panels  217 ,  218  may have thicknesses that vary over a length and/or width of the steel slab  200 . 
     Together, the first panel  217 , the second panel  218 , and the strut structure  230  of the illustrated embodiment form a composite panel. The composite panel may also be referred to as a sandwich panel. In some embodiments, the first panel  217 , the second panel  218 , and the strut structure  230  may be integrally formed as a single unit. For example, the steel slab  200  may be formed using any of the methods described in U.S. Pat. Nos. 9,623,480 and 10,493,522. In other embodiments, the first panel  217 , the second panel  218 , and the strut structure  230  may be formed as separate pieces that are secured together in any suitable manner, such as by welding. In such embodiments, at least the strut structure  230  may still be formed using any of the methods described in U.S. Pat. No. 9,623,480. In some embodiments, the strut structure  230  may be integrally formed with only one of the panels  217 ,  218 , and the other panel  217 ,  218  may be secured (e.g., welded) to the strut structure  230 . 
       FIGS. 4-6  illustrate a steel slab  300  prior to the hot rolling cycle according to another embodiment of the present invention. The illustrated steel slab  300  includes a body  304  having pores  320  that are empty voids shaped as triangular prisms. In the illustrated embodiment, the pores  320  are arranged in a series of pore rows  324  and pore columns  328  forming triangular prism voids and rectangular prism voids, similar to the pores  220  in the embodiment of  FIGS. 2 and 3 . Additionally, however, the pores  320  in the illustrated embodiment of  FIGS. 5 and 6  are arranged in multiple layers. The illustrated layers are arranged along a thickness of the body  304  of the steel slab  300 . The layers alternate between a panel layer  350  and a void layer  354 . The panel layer  350  defines a solid rectangular prism, similar to the first and second panels  217 ,  218  of  FIG. 2 . The illustrated steel slab  300  includes three panel layers  350  and two void layers  354  such that the void layers  354  are sandwiched between adjacent panel layers  350 . In additional embodiments, the steel slab  300  may include alternate numbers of the panel layers  350  and void layers  354  in any desired arrangement (e.g., two void layers  354  immediately adjacent one another, any number of additional alternative panel and void layers  350 ,  354 , and the like). 
       FIGS. 7 and 8  illustrate a steel slab  400  according to another embodiment of the present invention. The steel slab  400  includes a body  404  having pores  420  which form a truss structure or strut structure  430  between a first panel  417  and a second panel  418 , similar to the steel slab  200  of  FIG. 2 . The truss structure  430  is coupled to the first panel  417  and the second panel  418 . However, each strut  432  in the illustrated embodiment is either at an oblique angle relative to a plane created by the first panel  417  and the second panel  418  or is angled 90 degrees (i.e., perpendicular) relative to the plane. The struts  432  alternate between being at an oblique angle and being at 90 degrees relative to the plane. The struts  432  are distributed consistently and repeatedly along a length of the first panel  417  such that the truss structure  430  is uniform throughout the steel slab  400 . In other words, the struts  732  are arranged in a consistent pattern. In other embodiments, the truss structure  430  may be irregular throughout the steel slab  400 . Each strut  432  in the illustrated embodiment is a linear element, although in other embodiments some or all of the struts  432  can have other shapes connecting the first and second panels  417 ,  418 , such as curved, stepped, and/or tapered struts. Each strut  432  extends a length of the steel slab  400  (i.e., from a first end of the first and second panels  417 ,  418  to a second end of the first and second panels  417 ,  418 ). In some embodiments, the struts  432  may only extend a portion of the length of the steel slab  400 . In such embodiments, the other portion of the steel slab  400  may be solid or have a different strut. 
     The illustrated struts  432  form a non-auxetic structure between the first panel  417  and the second panel  418 . The non-auxetic truss structure  430  provides ballistic resistance. For example, a bullet or other projectile passing through the steel slab  400  generally passes through at least three “layers” of steel (e.g., the first panel  417 , the second panel  418 , and one of the struts  432 ), regardless of the angle of the bullet or other projectile. The non-auxetic truss structure  430  also reduces the weight of the steel slab  430 . For example, the illustrated steel slab  430  can weigh about 30% of a similar size, but solid steel slab. Each panel  417 ,  418  may have a thickness of at least 0.25 inches. In other embodiments, each panel  417 ,  418  may have a thickness of 0.5 inches or less. In the illustrated embodiment, each panel  417 ,  418  may have a thickness of about 0.35 inches. In some embodiments, the panels  417 ,  418  may have different thicknesses relative to each other. The steel slab  400  may have an overall thickness of at least 1 inch. In some embodiments, the steel slab  400  may have an overall thickness of at least 3 inches. In other embodiments, the steel slab  400  may have an overall thickness between 1 and 3 inches. 
     The struts  432  additionally include voids  436 . The illustrated voids  436  are positioned at evenly spaced rows and columns along the struts  432  such that the voids  436  are uniform throughout the steel slab  400 . The voids  436  allow material from, for example, a 3D printed insert (e.g., insert  1100  in  FIG. 22 ) to be interconnected during casting. The voids  436  may also help fragment and/or deflect a bullet or other projectile passing through the steel slab  400 . In the depicted embodiment, the voids  436  are oval in shape. In other embodiments, the voids  436  may have other shapes (e.g., circular, square, hexagonal, etc.). In still other embodiments, the voids  436  may not be present on every or any of the struts  432 . 
       FIGS. 9 and 10  illustrate a steel slab  500  according to another embodiment of the present invention. The steel slab  500  includes a body  504  having pores  520  which form a truss structure or strut structure  530  between a first panel  517  and a second panel  518 , similar to the steel slab  200  of  FIG. 2 . The truss structure  530  is coupled to the first panel  517  and the second panel  518 . However, each strut  532  in the illustrated embodiment is V shaped or Y shaped. In other words, the struts  532  include cross-sectional shapes that are V or Y shaped, as shown in  FIG. 10 . The struts  532  are arranged such that the struts  532  alternate between a V shaped strut  533  and a Y shaped strut  534 . The pores  520  are formed between subsequent struts  532 . The V shaped strut  533  is attached to solely the second panel  518  such that top points of the V shaped strut  533  are spaced from the first panel  517 . The Y shaped strut  534  is attached to the first panel  517  at top points of the Y and the second panel  518  at a bottom point of the Y. In other words, the V shaped strut  533  is upside down relative to the Y shaped strut  534 . The struts  532  are distributed consistently and repeatedly along a length of the first panel  517  such that the truss structure  530  is uniform throughout the steel slab  500 . In other words, the struts  532  are arranged in a consistent pattern. In other embodiments the truss structure  530  may be irregular throughout the steel slab  500 . Each strut  532 ,  533  extends a length of the steel slab  500  (i.e., from a first end of the first and second panels  517 ,  518  to a second end of the first and second panels  517 ,  518 ). In some embodiments, the struts  532 ,  533  may only extend a portion of the length of the steel slab  500 . In such embodiments, the other portion of the steel slab  500  may be solid or have different struts. 
     Similar to the struts  432  ( FIGS. 7 and 8 ), the illustrated struts  532  form a non-auxetic structure between the first panel  517  and the second panel  518 . As such, the non-auxetic truss structure  530  provides ballistic resistance and reduces a weight of the steel slab  500  by about 70%. Each panel  517 ,  518  may have a thickness of at least 0.25 inches. In other embodiments, each panel  517 ,  518  may have a thickness of 0.5 inches or less. In the illustrated embodiment, each panel  517 ,  518  may have a thickness of about 0.35 inches. In some embodiments, the panels  517 ,  518  may have different thicknesses relative to each other. The steel slab  500  may have an overall thickness of at least 1 inch. In some embodiments, the steel slab  500  may have an overall thickness of at least 3 inches. In some embodiments, the steel slab  500  may have an overall thickness of at least 6 inches. In other embodiments, the steel slab  500  may have an overall thickness between 1 and 6 inches. In still other embodiments, the steel slab  500  may have an overall thickness between 3 and 6 inches. 
     The struts  532  additionally include voids  536 . The illustrated voids  536  are positioned at evenly spaced rows and columns along the struts  532  such that the voids  536  are uniform throughout the steel slab  500 . The voids  536  allow material from, for example, a 3D printed insert (e.g., insert  1200  in  FIG. 23 ) to be interconnected during casting. In the depicted embodiment, the voids  536  are oval in shape. In other embodiments, the voids  536  may have other shapes (e.g., circular, square, hexagonal, etc.). In still other embodiments, the voids  536  may not be present on every or any of the struts  432 . 
       FIGS. 11 and 12  illustrate a steel slab  600  according to another embodiment of the present invention. The steel slab  600  includes a body  604  having pores  620  which form a truss structure or strut structure  630  between a first panel  617  and a second panel  618 , similar to the steel slab  200  of  FIG. 2 . The strut structure  630  is coupled to the first panel  617  and the second panel  618 . Each strut  632  in the illustrated embodiment is V shaped or Y shaped. Additionally, the struts  632  in the illustrated embodiment of  FIGS. 9 and 10  are arranged in multiple layers. The illustrated layers are arranged along a thickness of the body  604  of the steel slab  600 . Each layer  640 ,  644  is similar to the truss structure  530  ( FIGS. 9 and 10 ) described above. The layers alternate between a first strut layer  640  and a second strut layer  644 . The first strut layer  640  is similar to the second strut layer  644 , however, the second strut layer  644  is inverted (e.g., rotated 180 degrees) relative to the first strut layer  640 . The illustrated steel slab  600  includes two of the second strut layers  644  and one of the first strut layers  640 . Each strut layer  640 ,  644  is interconnected by one or more horizontal struts  648 . The horizontal struts  648  are parallel to the first panel  617  and the second panel  618 , but each only extends a portion of a width of the steel slab  600 . In additional embodiments, the steel slab  600  may include alternate numbers of the first strut layers  640  and the second strut layers  644 . 
     Similar to the struts  432  ( FIGS. 7 and 8 ), the illustrated struts  632  form a non-auxetic structure between the first panel  617  and the second panel  618 . As such, the non-auxetic truss structure  630  provides ballistic resistance and reduces a weight of the steel slab  600  by about 70%. The multiple strut layers  640 ,  644  also provide additional layers for a bullet or other projectile to pass through. Each panel  617 ,  618  may have a thickness of at least 0.25 inches. In other embodiments, each panel  617 ,  618  may have a thickness of 0.5 inches or less. In the illustrated embodiment, each panel  617 ,  618  may have a thickness of about 0.35 inches. In some embodiments, the panels  617 ,  618  may have different thicknesses relative to each other. The steel slab  600  may have an overall thickness of at least 1 inch. In some embodiments, the steel slab  600  may have an overall thickness of at least 3 inches. In some embodiments, the steel slab  600  may have an overall thickness of at least 6 inches. In other embodiments, the steel slab  600  may have an overall thickness between 1 and 6 inches. In still other embodiments, the steel slab  600  may have an overall thickness between 3 and 6 inches. 
     The truss structures  430 ,  530 ,  630  of the steel slabs  400 ,  500 ,  600  shown in  FIG. 7-12  are integrally formed with the first panels  417 ,  517 ,  617  and the second panels  418 ,  518 ,  618 . In other embodiments, the truss structures  430 ,  530 ,  630  may be formed as separate pieces from one or both of the panels  417 ,  418 ,  517 ,  518 ,  617 ,  618  and then secured (e.g., welded) to the panels  417 ,  418 ,  517 ,  518 ,  617 ,  618 . 
       FIGS. 13-16  illustrate a steel slab  700  according to another embodiment of the present invention. The steel slab in  FIGS. 13 and 14  is substantially similar to the steel slab in  FIGS. 15 and 16  except the steel slab in  FIGS. 13 and 14  is thicker. As such, the steel slabs will be described together. 
     The steel slab  700  includes a body  704  having pores  720  which form a truss structure of strut structure  730  between a first panel  717  and a second panel  718 , similar to the steel slab  200  of  FIG. 2 . However, the steel slab  700  includes struts  732  having a cross-section that is hexagonal in shape. Each strut  732  in the illustrated embodiment is a linear element having a bend such that one strut  732  forms two sides of the hexagon. In other embodiments, some or all of the struts  732  can have other shapes connecting the first and second panels  717 ,  718 , such as curved, stepped, and/or tapered struts. Two struts form one hexagon, with the first panel  717  and the second panel  718  forming two sides of the hexagonal. In other words, the pores  720  are hexagonal in shape. In the depicted embodiment, the steel slab  700  includes four struts  732  forming two hexagons. In other embodiments, the steel slab  700  may include any number of struts  732 . The struts  732  are distributed consistently and repeatedly along a length of the first panel  717  such that the truss structure  730  is uniform throughout the steel slab  700 . In other words, the struts  732  are arranged in a consistent pattern. In other embodiments, the truss structure  730  may be irregular throughout the steel slab  700 . Each strut  732  extends a length of the steel slab  700  (i.e., from a first end of the first and second panels  717 ,  718  to a second end of the first and second panels  717 ,  718 ). In some embodiments, the struts  732  may only extend a portion of the length of the steel slab  700 . In such embodiments, the other portion of the steel slab  700  may be solid or have different struts. 
     In the depicted embodiment, a thickness of each of the struts  732  is equal to or less than a thickness of the first and second panels  717 ,  718 . In other embodiments, a thickness of each of the struts  732  may be greater than the thickness of the first and second panels  717 ,  718  (as shown in  FIGS. 15 and 16 ). Each panel  717 ,  718  may have a thickness of at least 0.25 inches. In other embodiments, each panel  517 ,  518  may have a thickness of 0.5 inches or less. In some embodiments, the panels  517 ,  518  may have different thicknesses relative to each other. The steel slab  500  may have an overall thickness of at least 1 inch. In some embodiments, the steel slab  500  may have an overall thickness of at least 3 inches. In some embodiments, the steel slab  500  may have an overall thickness of at least 6 inches. In other embodiments, the steel slab  500  may have an overall thickness between 1 and 3 inches. In still other embodiments, the steel slab  500  may have an overall thickness between 1 and 6 inches. 
     The struts  732  additionally include voids  736 . The illustrated voids  736  are positioned at evenly spaced rows and columns along the struts  732  such that the voids  736  are uniform throughout the steel slab  700 . The voids  736  allow material from, for example, a 3D printed insert (e.g., insert  1400  or  1500  in  FIGS. 25 and 26 ) to be interconnected during casting. In the depicted embodiment, the voids  736  are oval in shape. In other embodiments, the voids  736  may have other shapes (e.g., circular, square, hexagonal, etc.). In still other embodiments, the voids  736  may not be present on every or any of the struts  732 . 
     The truss structure  730  of the steel sheet  700  shown in  FIGS. 13-16  is integrally formed with the first panel  717  and the second panel  718 . In other embodiments, the truss structure  730  may be formed as a separate piece from one or both of the panels  717 ,  718 . In such embodiments, the truss structure  730  may be secured (e.g., welded) to one of both of the panels  717 ,  718 . 
     The truss structure  730  forms an auxetic structure between the first panel  717  and the second panel  718 . Auxetic structures have a negative Poisson&#39;s ratio. When the auxetic structure is stretched, the structure becomes thicker perpendicular to an applied force. This occurs due to the shape of the auxetic structure deforming when a force is imparted onto the auxetic structure. For example, the octagonal shape of the truss structure  730  allows the truss structure  730  to extend when a blast forces the first and second panels  717 ,  718  apart. This extension of the truss structure  730  allows the truss structure  730  to absorb energy imparted onto the steel sheet  700  by the blast. The absorption of energy provides the steel sheet  700  greater blast resistance than a conventional, solid steel sheet. 
       FIG. 17  illustrates a hot-rolled steel sheet  100   b  according to an embodiment of the present invention. The hot-rolled steel sheet  100   b  is the steel slab  100  shown in  FIG. 1  after the steel slab  100  has undergone a hot rolling process as disclosed herein. The steel sheet  100   b  has the same chemical composition as described above in relation to the steel slab  100 . The steel sheet  100   b  includes a steel body  104   b  that is similar in shape to the steel body  104 , but has been flattened. When compared with the first face  108  and the second face  112  of the steel slab  100 , a first face  108   b  and a second face  112   b  of the steel sheet  100   b  have greater surface areas. A thickness of a peripheral edge  116   b  (i.e., a distance between the first face  108   b  and the second face  112   b ) of the steel sheet  100   b  is less than a thickness of the peripheral edge  116  of the steel slab  100 . In some embodiments, the thickness of the peripheral edge  116   b  may be reduced between 25% and 75% relative to the thickness of the peripheral edge  116 . In some embodiments, the thickness of the peripheral edge  116   b  may be reduced at least 25%. In other embodiments, the thickness of the peripheral edge  116   b  may be reduced at least 50%. In yet other embodiments, the thickness of the peripheral edge  116   b  may be reduced at least 75%. In some embodiments, the thickness of the peripheral edge  116   b  may be reduced less than 75%. In further embodiments, the thickness may be reduced 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or 75%. The illustrated peripheral edge  116   b  has a thickness within a range of 0.25 inches to 0.75 inches. However, in some embodiments, the thickness may be less than 0.25 inches or greater than 0.75 inches. 
     The steel body  104   b  also includes a plurality of interconnected pores  120   b  forming a substantially uniform pattern within the body  104   b . In some embodiments, the pores  120   b  have generally the same shape and configuration as the pores  120  described above in connection with  FIG. 1 , but are flattened during the hot rolling process. The pores  120   b  may also still be interconnected as also described above in connection with  FIG. 1 . In some embodiments, some of the pores  120   b  may no longer be interconnected due to the flattening process. In other embodiments, all or substantially all of the pores  120   b  may no longer be interconnected (e.g., each of the pores  120   b  may be isolated from adjacent pores  120   b ) due to the flattening process that collapses the channels between adjacent pores  120   b . After hot rolling, at least some of the plurality of interconnected pores  120   b  have substantially oval shaped cross-sections. In other embodiments, the pores  120   b  may have other flattened shapes (e.g., flattened oblong shapes, football-shaped, flattened cubic shapes, flattened polygonal shapes, etc.), depending at least in part upon the starting shapes of the pores  120 . Further, some of the pores  120   b  may be flattened to different degrees relative to other pores  120   b . By way of example only, pores  120   b  closer to a center of the body  104   b  may be more flattened than pores  120   b  closer to the peripheral edge  116   b  of the body  104   b , or vice versa. 
     The steel slabs of  FIGS. 1-6  may be, but do not necessarily need to be, hot rolled into hot-rolled steel sheets  100   b ,  200   b ,  300   b  of  FIGS. 17-19 .  FIGS. 7-16  may also be, but do not necessarily need to be, hot rolled into hot-rolled steel sheets. The pores of the hot-rolled steel sheets  100   b ,  200   b ,  300   b , can include generally the same shapes as the pores  120 ,  220 ,  320  described above, but with flattened profiles (resulting from the hot rolling process) when viewed in a cross-sectional plane oriented perpendicularly with respect to the opposite exterior surfaces of the sheet  100   b ,  200   b ,  300   b . Accordingly, the hot-rolled steel sheets  100   b ,  200   b ,  300   b  can have a plurality of flattened interconnected pores forming a substantially uniform pattern within the sheets  100   b ,  200   b ,  300   b . Like the steel slabs from which the steel sheets  100   b ,  200   b ,  300   b  are formed, the substantially uniform pattern can be a generally two-dimensional pattern (a grid, array, or lattice, by way of example only) that can define a rectangular pattern within the body of the steel sheets  100   b ,  200   b ,  300   b . In other embodiments, the substantially uniform pattern can be a three-dimensional pattern that can also define a rectangular or prismatic pattern within the body of the steel sheets  100   b ,  200   b ,  300   b . In other embodiments, the two-dimensional or three-dimensional arrangement of pores do not define an identifiable pattern as just described, yet still define a network of interconnected pores within the steel sheets  100   b ,  200   b ,  300   b.    
     As described above, the pores  120 ,  220 ,  320  can become flattened (i.e., having a flattened profile) from the hot rolling process also described herein. As a result, the pores  120 ,  200 ,  320  can each take on a shape in which each pore has length and/or width dimensions (in the plane of the steel sheet  100   b ,  200   b ,  300   b ) that are larger than the thickness dimensions (in a direction that is perpendicular to the steel sheet  100   b ,  200   b ,  300   b ). In some embodiments, at least a majority of the pores in the steel sheet  100   b ,  200   b ,  300   b  have this shape and orientation. Also in some embodiments, at least 80% of the pores in the steel sheet  100   b ,  200   b ,  300   b  have this shape and orientation. 
     Also as a result of the hot rolling process, the pores  120 ,  200 ,  320  can each take on a shape in which each pore has length (i.e., largest dimension) oriented generally in a common direction. In some embodiments, the common direction is the direction in which the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  was rolled, or in the length (i.e., largest dimension) direction of the steel slab and steel sheets  100   b ,  200   b ,  300   b . In some embodiments, at least a majority of the pores in the steel sheet  100   b ,  200   b ,  300   b  have this orientation. Also in some embodiments, at least 80% of the pores in the steel sheet  100   b ,  200   b ,  300   b  have this shape and orientation. 
     The pores in the resulting steel sheets  100   b ,  200   b ,  300   b  can still have the same general shape and/or relative arrangement as that in the steel slabs  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  from which the steel sheets  100   b ,  200   b ,  300   b  were formed, albeit with a generally flattened profile as described above. Accordingly, pores  120  having different shapes interconnected as described above can still have different shapes and interconnections. Similarly, pores  120 ,  220 ,  320  having two or more different pore sizes within the steel slabs  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  can still have two more different flattened pore sizes within the steel sheets  100   b ,  200   b ,  300   b.    
       FIG. 20  is a flowchart depicting a method of hot rolling a steel slab. References below to the steel slab generally refer to the steel slabs from  FIGS. 1-6 . References below to the steel sheet generally refer to the hot-rolled steel sheets from  FIGS. 17-19 , which depicts the steel slabs  100 ,  200 ,  300  after the steel slabs  100 ,  200 ,  300  have been hot-rolled. The slabs  400 ,  500 ,  600 ,  700  after the steel slabs  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  have been hot-rolled are not depicted. Although the method is described with reference to certain steps, not all of the steps need to be performed or need to be performed in the order presented. It will be appreciated that the method discussed below is equally applicable to lightweight steel slabs including other pore shapes, pore sizes, and pore arrangements as discussed herein. 
     At Step  900 , the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  is prepared by, for example, sand casting molten steel. Possible methods of preparing the steel slab  100  are disclosed in U.S. Pat. Nos. 9,623,480 and 10,493,522, the entire contents of which are incorporated by reference herein. As described above, the steel slab  100  includes the plurality of interconnected pores  120  that can form a substantially uniform pattern within the steel slab  100 . In some embodiment, the interconnected pores  120  are formed through the method of casting the molten steel into the steel slab  100 . In some embodiments, the interconnected pores  120  are formed as a plurality of spheres and cylinders. To prepare the steel slab of  FIG. 2 , a similar method described in the &#39;480 and &#39;522 Patents is used with an insert  1000  shown in  FIG. 21 . To prepare the steel slab of  FIG. 4 , two of the inserts  1000  may be used to create the pore layers  350  between the panel layers  354 . The inserts  1000  may be separated by, for example, one or more shims when positioned within a mold. In further embodiments, more than two inserts  1000  may be used to create a steel slab with more than two pore layers. To prepare the steel slab of  FIG. 4 , a similar method described in the &#39;480 and &#39;522 Patents is used with an insert  1100  shown in  FIG. 22 . To prepare the steel slab of  FIG. 8 , a similar method described in the &#39;480 and &#39;522 Patents is used with an insert  1200  shown in  FIG. 23 . To prepare the steel slab of  FIG. 10 , a similar method described in the &#39;480 and &#39;522 Patents is used with an insert  1300  shown in  FIG. 24 . To prepare the steel slab of  FIG. 12 , a similar method described in the &#39;480 and &#39;522 Patents is used with an insert  1400  shown in  FIG. 25 . To prepare the steel slab of  FIG. 14 , a similar method described in the &#39;480 and &#39;522 Patents is used with an insert  1500  shown in  FIG. 26 . 
     After the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  has been prepared, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  is cooled (Step  904 ). In some embodiments, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  may be cooled to 600 degrees Celsius. The steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  is cooled within, for example, six hours of completing preparation of the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700 . In other embodiments, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  may be cooled to other suitable temperatures, or within other time periods. 
     After the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  has been cooled in the illustrated embodiment, an antiscale coating is applied to the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  (Step  908 ). The antiscale coating may be applied to interior and exterior surfaces of the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  to reduce or minimize scaling of the interconnected pores  120  (Step  908 ). In some embodiments, this step may be omitted. 
     With continued reference to the illustrated embodiment of  FIG. 20 , following applying the antiscale coating, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  is reheated (Step  912 ). In some embodiments, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  may be reheated to a temperature of 1050 to 1230 degrees Celsius. In other embodiments, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  may be reheated to other suitable temperatures. Reheating may occur in, for example, a soaking oven. The steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  can be soaked in the soaking oven for a time of 1.5 hours per inch of thickness of the slab. For example, in some embodiments, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  has a thickness of 1 inch to 2 inches, which corresponds to a soaking time of 1.5 hours to 3 hours. The soaking oven may be a gas-fired or oil-fired soaking oven. In additional embodiments, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  may be heated through induction heating, or in other suitable manners. 
     At Step  916 , the reheated steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  begins a hot rolling cycle. The starting temperature of the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  as it begins the hot rolling cycle may be between 1050 degrees Celsius and 1230 degrees Celsius. In some embodiments, the hot rolling cycle begins by passing the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  between a first roller and a second roller of a mill. The first roller and the second roller can rotate in opposing directions, thereby moving the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  through the rollers. The steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  can be passed through the first roller and the second roller in a range of two to six passes, or may be passed through multiple sets of rollers, forming the steel sheet  100   b . The number of passes the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700  undergoes can correspond to a reduction of thickness of the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700  of between 25% and 75%. In some embodiments, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700  starts the hot rolling cycle at a thickness in a range of 1 inch to 2 inches, and the steel sheet  100   b  finishes hot rolling at a thickness in a range of 0.25 of an inch to 0.75 of an inch. In other embodiments, the steel slab  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700  may have other starting thicknesses, and/or the steel sheet  100   b ,  200   b ,  300   b  may have other finishing thicknesses. 
     Through the process of hot rolling, in some embodiments the interconnected pores  120  are flattened. In the illustrated embodiment of  FIG. 1 , the pores  120  are flattened such that at least some of the plurality of pores  120   b  have substantially oval-shaped cross-sections. In some embodiments, each of the plurality of pores  120   b  will have substantially similar oval-shaped cross-sections, while the plurality of pores in additional embodiments may include cross-sections that vary in size and shape. In the illustrated embodiments of  FIGS. 2 and 4 , the pores  220 ,  320  are flattened such that at least some of the plurality of pores have flattened triangular-shaped cross-sections. 
     At Step  920 , the hot rolling process ends. In some embodiments, a final temperature of the steel sheet  100   b ,  200   b ,  300   c  as the steel sheet completes hot rolling is greater than or equal to 850 degrees Celsius. In the event that the steel sheet  100   b ,  200   b ,  300   b  reaches the final temperature prior to a desired thickness being reached, the steel sheet  100   b ,  200   b ,  300   c  can again be soaked in a soaking oven to increase the temperature of the steel sheet  100   b ,  200   b ,  300   b . Once the temperature of the steel sheet  100   b ,  200   b ,  300   b  has increased, the hot rolling cycle may continue. 
     At Step  924 , the steel sheet  100   b ,  200   b ,  300   b  is cooled. In some embodiments, cooling of the steel sheet  100   b ,  200   b ,  300   b  may begin within five seconds of completion of hot rolling. The cooling of the steel sheet  100   b ,  200   b ,  300   b  may be an uncontrolled cooling process, meaning the steel sheet  100   b ,  200   b ,  300   b  does not undergo further treatment to facilitate cooling. In other embodiments, the cooling process may be controlled. In one example, the steel sheet  100   b ,  200   b ,  300   b  is initially cooled at a cooling rate in the range of 2 degrees Celsius per minute to 10 degrees Celsius per minute until the steel sheet  100   b ,  200   b ,  300   b  reaches an ambient temperature of approximately 30 degrees Celsius. 
     At Step  928  of the illustrated embodiment, the steel sheet  100   b ,  200   b ,  300   b  is heat treated. In some embodiments, the steel sheet  100   b ,  200   b ,  300   b  may be heat treated through a quenching and tempering process. In other embodiments, the steel sheet  100   b ,  200   b ,  300   b  may be heat treated through a normalizing and tempering process. The steel sheet  100   b ,  200   b ,  300   b  can be heat treated to achieve a hardness in a range of, for example, 150 Brinell hardness number (BHN) to 400 BHN. In some embodiments, the steel sheet  100   b ,  200   b ,  300   b  may be heat treated to have a hardness of 150 BHN, 200 BHN, 250 BHN, 300 BHN, 350 BHN, or 400 BHN. In other embodiments, the steel sheet  100   b  may be heat treated to have a hardness less than 150 BHN or greater than 400 BHN. In further embodiments, this step may be omitted. 
     As noted above,  FIG. 21  illustrates the insert  1000  used for forming the strut structure  230  of the steel slabs  200 ,  300  shown in  FIGS. 2-6 . The illustrated insert  1000  includes two faces  1004 ,  1008  and a series of channels  1012  extending between the faces  1004 ,  1008 . The channels  1012  are voids of space matching the shapes of the struts  232  ( FIG. 2 ). As such, the channels  1012  may be formed to have different configurations (e.g., angles, lengths, thicknesses, etc.) depending on the desired configurations of the struts  232 . 
       FIG. 22  illustrates an insert  1100  used for forming the strut structure  430  of the steel slabs  400  shown in  FIGS. 7 and 8 . The illustrated insert  1100  includes two faces  1104 ,  1108  and a series of channels  1112  extending between the faces  1104 ,  1108 . The channels  1112  are voids of space matching the shapes of the struts  432  ( FIG. 8 ). As such, the channels  1112  may be formed to have different configurations (e.g., angles, lengths, thicknesses, etc.) depending on the desired configurations of the struts  432 . 
       FIG. 23  illustrates an insert  1200  used for forming the strut structure  530  of the steel slabs  500  shown in  FIGS. 9 and 10 . The illustrated insert  1200  includes two faces  1204 ,  1208  and a series of channels  1212  extending between the faces  1204 ,  1208 . The channels  1212  are voids of space matching the shapes of the struts  532  ( FIG. 10 ). As such, the channels  1212  may be formed to have different configurations (e.g., angles, lengths, thicknesses, etc.) depending on the desired configurations of the struts  532 . 
       FIG. 24  illustrates an insert  1300  used for forming the strut structure  630  of the steel slabs  600  shown in  FIGS. 11 and 12 . The illustrated insert  1300  includes two faces  1304 ,  1308  and a series of channels  1312  extending between the faces  1304 ,  1308 . The channels  1312  are voids of space matching the shapes of the struts  632  ( FIG. 12 ). As such, the channels  1312  may be formed to have different configurations (e.g., angles, lengths, thicknesses, etc.) depending on the desired configurations of the struts  632 . 
       FIG. 25  illustrates an insert  1400  used for forming the strut structure  730  of the steel slabs  700  shown in  FIGS. 13 and 14 . The illustrated insert  1400  includes two faces  1404 ,  1408  and a series of channels  1412  extending between the faces  1404 ,  1408 . The channels  1412  are voids of space matching the shapes of the struts  732  ( FIG. 14 ). As such, the channels  1412  may be formed to have different configurations (e.g., angles, lengths, thicknesses, etc.) depending on the desired configurations of the struts  732 . 
       FIG. 26  illustrates an insert  1500  used for forming the strut structure  730  of the steel slabs  700  shown in  FIGS. 15 and 16 . The illustrated insert  1500  includes two faces  1504 ,  1508  and a series of channels  1512  extending between the faces  1504 ,  1508 . The channels  1512  are voids of space matching the shapes of the struts  732  ( FIG. 16 ). As such, the channels  1512  may be formed to have different configurations (e.g., angles, lengths, thicknesses, etc.) depending on the desired configurations of the struts  732 . 
     The insert  1000 ,  1100 ,  1200 ,  1300 ,  1400 ,  1500  can be used in a sand casting process, such as the process disclosed in U.S. Pat. Nos. 9,623,480 and 10,493,522. As such, the insert  1000 ,  1100 ,  1200 ,  1300 ,  1400 ,  1500  may formed using similar materials and processes disclosed in the &#39;480 and &#39;522 Patents, such as 3-D printing. 
     The above techniques allow for the creation of steel sheets or composite panels, with ballistic-resistant application for, for example, military structures (e.g., ballistic plates), civilian structures (e.g., building and bridges), naval applications, and the like. In some scenarios, the steel sheets may be used in ships or aircrafts. The steel sheets or composite panels also have applications in energy absorption, blast resistance, and sound absorption. The lightweight steel slabs used to cast the steel sheets allow for large steel sheets to be produced cost-effectively, as larger thickness sections of the steel slabs may be used. Structural advantages of lightweight steel sheets compared to solid steel sheets include minimization of weight, maximization of flexural strength, increased energy dissipation, and increased damping. 
     Non-auxetic structures (such as the steel slabs shown in  FIGS. 2-12 ) are very good for ballistic resistance by having more material (e.g., plates) in the line of a shot when a bullet or other projectile hits. Controlling the chemistry and heat treatment to give higher hardness gives higher ballistic resistance. In contrast, auxetic structures (such as the steel slabs shown in FIGS.  13 - 16 ) are found to have higher blast resistance due to their negative Poisson&#39;s ratio and ability to collapse under the influence of blast pressure. Lower hardness tends to favor blast resistance, while higher hardness tends to favor ballistic resistance. Controlling the chemistry and heat treatment allows production of both ballistic resistant and blast resistant lightweight steel sheets with areal densities comparable to similar thickness aluminum plates. 
     Although the steel slabs  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  are all described above as going through a hot-rolling process, in some scenarios, the steel slabs  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  400 ,  500 ,  600 ,  700  may be used as-is without hot-rolling. If the steel slabs are not hot rolled, the steel slabs can still be used as steel sheets or as other components. For example, some applications may require a combination of panels that are both hot-rolled and not hot-rolled. Alternatively, some applications may only require panels that are not hot-rolled. 
     Various features and advantages of the invention are set forth in the following claims.