Product made of high-strength hot-rolled steel sheet

Abstract

This invention relates to a high-strength rolled steel sheet product and a method for producing a high-strength rolled steel sheet product. The present invention provides steel sheet products with controlled compositions that are subjected to two-stage annealing processes to produce sheet products with desirable microstructures and favorable mechanical properties, such as high strength and super-high plasticity. The steel sheet products can be cold-rolled or hot-rolled. The steels processed according to the present invention exhibit favorable combined tensile strength and total elongation (uts·te) properties and can fall into the category of advanced generation 3 high-strength steels, desirable in various industries, including automobile manufacturers.

Description

24 PRODUCT MADE OF HIGH-STRENGTH HOT-ROLLED STEEL SHEET CROSS-REFERENCE TO RELATED REQUESTS

[001] This application is a continuation in part of the Patent Application for U.S. Patent No. 15 / 591,344, filed May 10, 2017, now U.S. Patent No. 10,385,419, which claims priority to U.S. Provisional Application No. 62 / 334,189 filed May 10, 2016, and U.S. Provisional Application No. 62 / 396,602 filed September 19, 2016. This application is also a continuation in part of U.S. Patent Application No. 16 / 459,757, filed July 2, 2019, which is a continuation of U.S. Patent Application No. 15 / 591,344. All prior applications are incorporated herein by reference. FIELD OF THE INVENTION

[002] The present invention relates to high-strength steel products with favorable properties and to annealing processes for manufacturing such products. FUNDAMENTALS

[003] In recent years, the global steel industry has focused on developing a third generation of advanced high-strength steel (AHSS) for the automotive market. These Generation 3 steels have a favorable balance of tensile strength and elongation, typically in a UTS*TE range around 20,000 MPa% or higher. However, the steel industry has had difficulty commercializing Generation 3 AHSS, as most approaches require high alloy content, for example, typically greater than 4% by weight of manganese, resulting in difficulties in manufacturing these steels with conventional steelmaking equipment. Furthermore, currently available AHSS has been difficult to weld using techniques such as tack welding, and has been difficult to coat with galvanic coatings. Petition 870260020852, dated 06 / 03 / 2026, page 12 / 65 / 24 of zinc and has been difficult to manufacture in a thin gauge sheet necessary for large-scale application. SUMMARY OF THE INVENTION

[004] The present invention provides steel sheet products with controlled compositions that are subjected to two-stage annealing processes to produce sheet products with desirable microstructures and favorable mechanical properties, such as high strength and super-high plasticity. The steel sheet products can be cold-rolled or hot-rolled. The steels processed according to the present invention exhibit favorable tensile strength and total elongation (UTS'TE) properties combined, for example, greater than 25,000 MPa% when tested using standard reduced ASTM or full-size JIS tensile test procedures. Furthermore, the steels produced according to the present invention exhibit favorable combinations of TE and hole expansion, i.e., both overall plasticity and local plasticity are good.Steels with these properties fall into the category of Generation 3 advanced high-strength steels and are highly sought after by various industries, including automobile manufacturers.

[005] One aspect of the present invention is to provide a high-strength rolled steel sheet product comprising 0.12 to 0.5 percent by weight of C, 1 to 3 percent by weight of Mn and 0.8 to 3 percent by weight of a combination of Si and Al, wherein the steel sheet product has been subjected to a two-stage annealing process, comprises substantially equiaxed ferrite and retained austenite grains with an average aspect ratio of less than 3:1 and has a combined tensile strength and total elongation UTS^TE greater than 25,000 MPa%.

[006] Another aspect of the present invention is to provide a method for Petition 870260020852, dated 06 / 03 / 2026, page 13 / 65 / 24 to produce a high-strength rolled steel sheet product comprising 0.12 to 0.5 percent by weight of C, 1 to 3 percent by weight of manganese and 0.8 to 3 percent by weight of a combination of Si and Al. The method comprises subjecting the steel sheet product to a first-stage annealing process to achieve a predominantly martensitic microstructure and subjecting the steel sheet product to a second-stage process comprising soaking the sheet product in an intercritical regime at a temperature of 720 to 850°C, followed by maintaining the sheet product at a temperature of 370 to 445°C.

[007] These and other aspects of the present invention will become more apparent from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[008] Fig. 1 includes temperature versus time graphs illustrating a two-stage annealing process according to an embodiment of the present invention.

[009] Fig. 2 includes temperature versus time graphs illustrating a two-stage annealing process according to another embodiment of the present invention.

[0010] Fig. 3 is a temperature versus time graph illustrating a two-stage annealing process that combines the two-stage thermal process with an optional zinc-based hot-dip coating operation in a single production facility.

[0011] Fig. 4 is a temperature versus time graph for a second stage of an annealing process defining soaking and holding zones in the thermal cycle according to an embodiment of the invention.

[0012] Figs. 5 and 6 are electron diffraction photomicrographs. Petition 870260020852, dated 06 / 03 / 2026, page 14 / 65 / 24 backscattered (EBSD) images illustrating the microstructure of a high-strength steel sheet product according to an embodiment of the invention.

[0013] Fig. 7 is an optical photomicrograph of a steel sheet product subjected to the thermal process shown in Fig. 1, showing darker ferrite grains and lighter austenite grains.

[0014] Fig. 8 is a bar graph illustrating the aspect ratios of the austenite grains shown in Fig. 7.

[0015] Figs. 9 and 10 are graphs for a high-strength steel sheet product showing austenite and ferrite grain size distributions according to an embodiment of the invention.

[0016] Fig. 11 is an EBSD photomicrograph illustrating the microstructure of a high-strength steel sheet product processed as shown in Fig. 1.

[0017] Figs. 12 and 13 are EBSD photomicrographs showing processed steel sheet products as shown in Fig. 2.

[0018] Fig. 14 is an EBSD photomicrograph of a steel sheet product processed as shown in Fig. 3.

[0019] Fig. 15 is a graph of total elongation versus tensile strength limit for high-strength steel sheet products of the present invention compared with other steel sheet products processed outside the scope of the present invention.

[0020] Fig. 16 is a graph of total elongation versus tensile strength limit for high-strength steel products produced in crush tests according to embodiments of the present invention.

[0021] Fig. 17 is a temperature versus time graph for cold-rolled and hot-rolled substrates processed with a thermal cycle according to an embodiment of the invention. Petition 870260020852, dated 06 / 03 / 2026, page 15 / 65 / 24

[0022] Fig. 18 is an EBSD photomicrograph of a cold-rolled steel sheet substrate subjected to the thermal process shown in Fig. 17, showing darker ferrite grains and lighter retained austenite grains.

[0023] Fig. 19 is an EBSD photomicrograph of a hot-rolled steel sheet substrate subjected to the thermal process shown in Fig. 17, showing darker ferrite grains and lighter retained austenite grains. DETAILED DESCRIPTION

[0024] The high-strength steel sheet products of the present invention have controlled compositions which, in combination with controlled annealing processes, produce desirable microstructures and favorable mechanical properties, including high strengths and super-high plasticities. In certain embodiments, the steel composition may include carbon, manganese, and silicon, along with any other suitable alloying additions known to those skilled in the art. Examples of steel compositions including ranges of C, Mn, Si, Al, Ti, and Nb are listed in Table 1 below. Table 1 Steel Compositions (% by weight) Example C Mn Si Al Si + AL Ti Nb A 0.12-0.5 1-3 0-2 0-2 0.8-3 0-0.05 0-0.05 B 0.15-0.4 1.3-2.5 0.2-1.8 0-1.5 0.9-2.5 0-0.03 0-0.03 C 0.17-0.35 1.5-2.3 0.4-1.5 0-1 1-2 0-0.02 0-0.02

[0025] In addition to the quantities of C, Mn, Si, Al, Ti and Nb listed in Table 1, steel compositions may include minor amounts or impurities of other elements, such as a maximum of 0.015 of S, a maximum of 0.03 of P, a maximum of 0.2 of Cu, a maximum of 0.02 of Ni, a maximum of 0.2 of Cr, a maximum of 0.2 of Mo, a maximum of 0.1 of Sn, a maximum of 0.015 of N, a maximum of 0.1 of V, and a maximum of 0.004 of B. As used in this document, the term “substantially free” refers to the composition of the sheet product. Petition 870260020852, dated 06 / 03 / 2026, p. 16 / 65 / 24 steel, means that a certain element or material is not intentionally added to the composition and is present only as an impurity or in trace amounts.

[0026] In the thin sheet steel products of the present invention, C provides increased strength and promotes the formation of retained austenite. Mn provides hardening and acts as a solid solution and strengthening agent. Si inhibits iron carbide precipitation during heat treatment and increases austenite retention. Al inhibits iron carbide precipitation during heat treatment and increases austenite retention. Ti and Nb can act as grain refiners that enhance strength.

[0027] In certain embodiments, Al may be present in an amount of at least 0.1 percent by weight or at least 0.2 percent by weight. For example, Al may be present in an amount of 0.5 to 1.2 percent by weight, or 0.7 to 1.1 percent by weight, in certain embodiments. Alternatively, the steel sheet product may be substantially free of Al.

[0028] Steel sheet products with the compositions described above are subjected to a two-stage annealing process, as described in more detail below. The resulting sheet products have been found to possess favorable mechanical properties, including desirable tensile strength limits, high elongations, high lambda values, high flexibility capacity, and high yield ratios (YS / UTS).

[0029] In certain embodiments, the tensile strength limit (TSL) of steel sheet products ranges from 700 to 1,100 MPa or more. In certain embodiments, the steel sheet product has a tensile strength limit greater than 700 MPa, for example, from 720 to 1,100 MPa, or from 750 to 1,050 MPa. Petition 870260020852, dated 06 / 03 / 2026, page 17 / 65 / 24

[0030] In certain embodiments, steel sheet products typically have a total elongation (TE) greater than 22 percent, for example, greater than 27 percent or greater than 33 percent. For example, the steel sheet product may have a total elongation of at least 20% or at least 25% or at least 27%, for example, from 22 to 45% or from 25 to 40%.

[0031] Steel sheet products may have lambda (λ) values ​​measured by a standard hole expansion test typically greater than 20 percent, for example, greater than 25 percent, or greater than 30 percent, or greater than 35 percent. The total expansion ratio or lambda may be greater than 20%, for example, from 22 to 80% or from 25 to 60%.

[0032] In certain embodiments, increased values ​​of total elongation (TE) and hole expansion (λ) result in sheet steel products exhibiting good overall plasticity and local plasticity.

[0033] Resistance stretching equilibria (UTS^TE) greater than 25,000 are observed for current steel sheet products, placing them in the category of Generation 3 steels highly sought after by industries such as the automotive industry. In certain applications, UTS^TE values ​​can be greater than 27,000, or greater than 30,000, or greater than 35,000.

[0034] According to certain embodiments of the invention, the final microstructure of the steel sheet products may comprise mainly ferrite, for example, at least 50% up to 80% or more, with smaller amounts of retained austenite, for example, from 5 to 25%, and small amounts of initial martensite, for example, from 0 to 10% or 15%. The amounts of ferrite, austenite, and martensite can be determined by standard EBSD techniques. Alternatively, the retained austenite content can be determined by magnetic saturation methods. Unless otherwise specified in this document, Petition 870260020852, dated 06 / 03 / 2026, page 18 / 65 / 24 the percentage by volume of retained austenite is determined by the EBSD technique.

[0035] In certain embodiments, retained austenite comprises from 1 to 25 percent by volume, for example, from 5 to 20 percent by volume. The amount of initial martensite may comprise less than 15 percent by volume, or less than 10 percent by volume, or less than 5 percent by volume. In certain embodiments, the steel sheet product is substantially free of initial martensite. It has been found that when the amounts of initial martensite are greater than 15%, the hole expansion values ​​decrease significantly, for example, the local plasticity is significantly reduced.

[0036] At least a portion of the ferrite may be formed during the heating section, as described below, by recrystallization and / or tempering of martensite, or during the cooling and holding section of the second annealing process by austenite decomposition. Part of the ferrite may be considered bainitic ferrite. The ferrite, austenite, and martensite phases are fine-grained, for example, with average grain sizes less than 10 microns, for example, less than 5 microns or less than 3 microns. For example, the ferrite grain size may vary from less than 10 microns, for example, less than 8 microns or less than 6 microns. The average austenite grain size may vary from less than 2 microns, for example, less than 1 micron or less than 0.5 microns. The grain size of artensite, when present, can vary from less than 10 microns, for example, less than 8 microns or less than 6 microns.

[0037] Austenite grains can be substantially equiaxed, for example, with average ratios less than 3:1 or less than 2:1, for example, around 1:1. It has been found that quantities of retained austenite Petition 870260020852, dated 06 / 03 / 2026, page 19 / 65 / 24 below around 5% result in significantly reduced total elongations (TE). It was further found that retained austenite quantities above 25% can only be obtained at very high carbon levels, resulting in poor weldability.

[0038] In certain embodiments of the invention, a two-stage annealing process is used to produce advanced high-strength steel products with favorable mechanical properties, such as those described above. Within each of the first and second annealing stages, various methodologies for carrying out the heat treatment can be used. Examples of two-stage annealing processes are shown in Figs. 1 to 3 and described below. Fig. 1 represents a continuous annealing line (CAL) followed by a continuous annealing line (CAL) production route. Fig. 2 represents a CAL plus continuous galvanizing line (CGL) production route. Fig. 3 represents a specially designed line allowing the CAL + CAL or CAL + CGL stages to occur in a single installation. While a direct firing furnace (DFF) followed by a radiant tube furnace (RT) embodiment is shown in Fig.3. Other methods, such as full-tube heating, electric radiant heating, and similar methods, can be used to achieve the desired thermal cycles. Step 1

[0039] The objective of the first stage of the annealing process is to achieve a martensitic microstructure in cold-rolled or hot-rolled steel sheet. In the first stage of annealing, an annealing temperature above the A3 temperature can typically be used, for example, an annealing temperature of at least 820°C can be used. In certain embodiments, the annealing temperature of the first stage can typically range from 830 to 980°C, for example, from 830 to 940°C, or from 840 to Petition 870260020852, dated 06 / 03 / 2026, page 20 / 65 / 24 930°C, or from 860 to 925°C. In certain embodiments, the peak annealing temperature can normally be maintained for at least 20 seconds, for example, from 20 to 500 seconds, or from 30 to 200 seconds. Heating can be achieved by conventional techniques such as a non-oxidizing or oxidizing direct firing furnace (DFF), oxygen-enriched DFI, induction, gas-fired radiant tube heating, electric irradiation heating, and the like. Examples of heating systems that can be adapted for use in the processes of the present invention are described in U.S. Patents Nos. 5,798,007; 7,368,689; 8,425,225; and 8,845,324, U.S. Patent Application No. 2009 / 0158975, and Published PCT Application No. WO / 2015083047, assigned to Fives Stein. Additional examples of heating systems that can be adapted for use in the processes of the present invention include U.S. Patent No. 7,384,489 assigned to Drever International, and U.S. Patent No. 9,096.918 attributed to Nippon Steel and Sumitomo Metal. Corporation. Any other known suitable types of heating systems and processes may be adapted for use in Stage 1 and Stage 2.

[0040] In the first stage, after the peak annealing temperature has been reached and maintained for the desired period of time, the cold-rolled or hot-rolled steel sheet is quenched at room temperature, or at a controlled temperature above room temperature, as described in more detail below. The quenching temperature may not necessarily be room temperature, but must be below the initial martensite temperature (Ms), and preferably below the final martensite temperature (MF), to form a predominantly martensite microstructure. In certain embodiments, between the first and second stages of the process, the thin steel sheet product may be cooled to a temperature below 300°C, for example, below 200°C. Petition 870260020852, dated 06 / 03 / 2026, p. 21 / 65 / 24

[0041] Tempering can be achieved by conventional techniques such as water quenching, submerged blade / nozzle quenching, gas quenching, rapid quenching using a combination of cold, warm or hot water and gas, quenching with water solution, quenching with other liquids or gaseous fluids, cooled roll quenching, water vapor spraying, wet flash quenching, non-oxidizing wet flash quenching, and the like. A quenching rate of 30 to 2,000°C / s can normally be used.

[0042] Various types of quenching and cooling systems and processes known to those skilled in the art can be adapted for use in the processes of the present invention. Suitable cooling / quenching systems and processes conventionally used on a commercial basis may include water quenching, steam quenching, dry flash and wet flash, oxidizing and non-oxidizing quenching, alkane-to-gas phase change quenching, hot water quenching, including two-stage water quenching, roller quenching, high-percentage hydrogen or helium jet quenching, and the like. For example, dry flash and / or wet flash oxidizing and non-oxidizing quenching may be used, as described in PCT application No. WO2015 / 083047 published by Fives Stein.Other patent documents by Fives Stein describing cooling / quenching systems and processes that can be adapted for use in the processes of the present invention include U.S. Patents Nos. 6,464,808B2; 6,547,898B2; and 8,918,199B2, and U.S. Patent Application Publications Nos. US2009 / 0158975A1; US2009 / 0315228A1; and US2011 / 0266725A1. Other examples of cooling / quenching systems and processes that can be adapted for use in the present invention include U.S. Patents Nos. 8,359,894B2; Petition 870260020852, dated 06 / 03 / 2026, page 22 / 65 / 24 8,844,462B2; and 7,384,489B2, and U.S. Patent Application Publications No. 2002 / 0017747A1 and 2014 / 0083572A1.

[0043] In certain embodiments, after the peak annealing temperature of the first stage is reached and the steel is quenched to form martensite, the martensite may optionally be tempered to soften the steel somewhat to make further processing more feasible. Tempering occurs by raising the temperature of the steel in the ambient temperature range to around 500°C and holding it for up to 600 seconds. If tempering is used, the tempering temperature may be kept constant, or it may be varied within this preferred range.

[0044] After tempering, the temperature is reduced to ambient temperature. The rate of this ramp-up decrease can typically vary from 1 to 40°C / s, for example, from 2 to 20°C / s. In the case of a single-pass furnace installation, as in Fig. 3, tempering may not be necessary. Step 2

[0045] The second stage of the annealing process may include a first stage that is conducted at a relatively high annealing temperature and a second stage that is conducted at a relatively low temperature. These stages are defined as the “soaking” and “holding” zones of the second annealing, as described in Fig. 4. The temperatures are controlled in order to promote the formation of the desired microstructure and mechanical properties in the final product.

[0046] In the first annealing stage of the second stage, a soaking temperature between A1 and A3 may be used, for example, an annealing temperature of at least 720°C may be used. In certain embodiments, the soaking zone temperature may normally vary from 720 to 850°C, for example, from 760 to 825°C. In certain embodiments, the Petition 870260020852, dated 06 / 03 / 2026, p. 23 / 65 / 24 peak annealing temperature can normally be maintained for at least 15 seconds, for example, from 20 to 300 seconds, or from 30 to 150 seconds.

[0047] During the first stage of the second stage, the soaking zone temperature can be reached by heating the steel from a relatively low temperature below Ms, for example, ambient temperature, at an average rate of 0.5 to 50°C / s, for example, around 2 to 20°C / s. In certain embodiments, the ramp-up can take from 25 to 800 seconds, for example, from 100 to 500 seconds. Heating of the first stage of the second stage can be achieved by any suitable heating system or process, such as the use of radiant heating, induction heating, direct firing furnace heating and the like.

[0048] After the soaking zone temperature is reached and maintained for the desired period of time, the steel can be cooled to a controlled temperature above ambient temperature up to the holding zone. In certain embodiments, the steel sheet product is held at a temperature above 300°C between the second-stage soaking process and the second-stage holding process. Cooling from the soaking zone to the holding zone can be achieved by conventional techniques such as water cooling, gas cooling, and the like. An average cooling rate of 5 to 400°C / s can normally be used. Any suitable types of cooling and quenching systems can be adapted for use in cooling from the soaking temperature to the holding temperature, including those described above.

[0049] According to embodiments of the invention, the holding zone step is carried out at a typical temperature of 360 to 445°C, for example, 370 to 440°C. The holding zone can be maintained for up to 800 seconds, for Petition 870260020852, dated 06 / 03 / 2026, p. 24 / 65 / 24 example, from 30 to 600 seconds.

[0050] The temperature of the holding zone can be kept constant or can vary slightly within the preferred temperature range. After holding, the steel can be reheated, such as by induction or another heating method, to enter a hot-dip plating crucible at the appropriate temperature to obtain good plating results, if the steel is to be hot-dip plating.

[0051] In certain embodiments, after the temperature of the holding zone has been maintained for a desired period of time, the temperature can be reduced to ambient temperature. Such a ramp-down can typically take from 10 to 1,000 seconds, for example, from around 20 to 500 seconds. The rate of such a ramp-down can typically vary from 1 to 1,000°C / s, for example, from 2 to 20°C / s.

[0052] According to certain embodiments, one or both of the first and second stage annealing processes can be carried out in a continuous annealing line (CAL). After undergoing a CAL + CAL process, the steel can be electrogalvanized to produce a zinc-based coated product.

[0053] In certain embodiments, the annealed steel sheet is hot-dip galvanized at the end of the retention zone. Galvanizing temperatures can typically range from 440 to 480°C, for example, from 450 to 470°C. In certain embodiments, the galvanizing step can be carried out as part of the second-stage annealing process in a continuous galvanizing line (CGL), for example, as shown in Fig. 2. This CAL + CGL process can be used to produce either a hot-dip galvanized zinc-based or zinc alloy-based product, or reheated after coating to produce a galvanized and annealed ferro-zinc type coated product. An optional coating step based on Petition 870260020852, dated 06 / 03 / 2026, page 25 / 65 / 24: Nickel plating can occur between the CAL and CGL stages in the process to improve the properties of the zinc coating. The use of a continuous galvanizing line in the second stage increases the production efficiency of a coated GEN3 product compared to using a CAL + CAL + EG route.

[0054] A galvanized product or hot-dip coated product of zinc-based alloy can also be made in a specially designed CGL in which two-stage annealing can occur in a single line, as shown in Fig. 3. Galvanizing and annealing can also be an option in this case. Furthermore, a single production facility can also be specially designed and constructed to combine the two-stage thermal process to produce uncoated Generation 3 steels, as defined in the invention.

[0055] The following examples are intended to illustrate various aspects of the present invention and are not intended to limit the scope of the invention. Example 1

[0056] A cold-rolled steel sheet with a composition as listed in Table 2, Sample No. 1, was subjected to a two-stage annealing process, as illustrated in Fig. 1. The microstructure of the resulting product is shown in Figs. 5 and 6. EBSD techniques using the commercial EDAX orientation image microscopy software show the dark ferrite grains and light austenite grains in Fig. 5. Example 2

[0057] A cold-rolled steel sheet with a composition as listed in Table 2, Sample No. 2, was subjected to a two-stage annealing process, as illustrated in Fig. 1. The microstructure of the resulting product is shown in Fig. 11. The mechanical properties of Sample No. 2 are listed in Table 2. The austenite grain size distributions Petition 870260020852, dated 06 / 03 / 2026, page 26 / 65 / 24 and ferrite are shown in Figs. 9 and 10, respectively. The average grain size of austenite is less than 1 micron and the average grain size of ferrite is less than 10 microns.

[0058] The microstructure includes approximately 80 percent by volume of ferrite with an average grain size of about 5 microns, approximately 10 percent by volume of retained austenite with substantially equiaxed grains and an average grain size of about 0.5 microns, and approximately 10 percent by volume of initial martensite with an average grain size of about 5 microns. The mechanical properties of Sample No. 1 are listed in Table 2 below. Example 3

[0059] A cold-rolled steel sheet with a composition as listed in Table 2, Sample No. 3, was subjected to a two-stage annealing process, as illustrated in Fig. 2. The microstructure of the resulting product is shown in Figs. 12 and 13. In Fig. 13, the austenite is light in color and the ferrite is dark in color. The mechanical properties of Sample No. 3 are listed in Table 2. Example 4

[0060] A cold-rolled steel sheet with a composition as listed in Table 2, Sample No. 4, was subjected to a two-stage annealing process, as illustrated in Fig. 3. The microstructure of the resulting product is shown in Figure 14. In Fig. 14, austenite is light in color and ferrite is dark in color. The mechanical properties of Sample No. 4 are listed in Table 2. Example 5

[0061] A cold-rolled steel sheet with a composition as listed in Table 2, Sample No. 5, was subjected to a process of Petition 870260020852, dated 06 / 03 / 2026, page 27 / 65 / 24 two-stage annealing, as illustrated in Fig. 1. The mechanical properties of Sample No. 5 are listed in Table 2. Example 6

[0062] A cold-rolled steel sheet with a composition as listed in Table 2, Sample No. 6, was subjected to a two-stage annealing process, as illustrated in Fig. 1. The mechanical properties of Sample No. 6 are listed in Table 2. Fig. 7 is an optical image showing the microstructure of the steel shown in Fig. 2, Sample No. 6, which was subjected to the two-stage annealing process shown in Fig. 1. In Fig. 7, the dark regions of the photomicrograph are ferrite grains, while the light regions are austenite grains. Fig. 8 is a graph illustrating the aspect ratios of the austenite grains shown in Fig. 7. The optical image of Fig. 7 was used to determine the proportions of austenite grains using image analysis with commercially available software. Fig. 7 shows that the average aspect ratio is less than 3:1 for the austenite grains. Example 7

[0063] A cold-rolled steel sheet with a composition as listed in Table 2, Sample No. 7, was subjected to a two-stage annealing process, as illustrated in Fig. 2. The mechanical properties of Sample No. 7 are listed in Table 2. Example 8

[0064] A cold-rolled steel sheet with a composition as listed in Table 2, Sample No. 8, was subjected to a two-stage annealing process, as illustrated in Fig. 3. The mechanical properties of Sample No. 8 are listed in Table 2.

[0065] The steels in Examples 1 to 8 exhibited UTS levels in the range of 700 to 1,100 MPa. Petition 870260020852, dated 06 / 03 / 2026, page 28 / 65 / 24 Comparative Examples 1 to 4

[0066] Cold-rolled steel sheets with compositions listed in Table 2, Samples No. C1 to C4, were subjected to a two-stage annealing process, as illustrated in Fig. 1. The mechanical properties of Samples No. C1 to C4 are listed in Table 2. The steels in Comparative Examples 1 to 4 exhibited UTS levels below 700 MPa. Comparative Examples 5 to 8

[0067] Cold-rolled steel sheets with compositions listed in Table 2. Samples No. C5 to C8 were subjected to a two-stage annealing process, as illustrated in Fig. 1. The mechanical properties of Samples No. C5 to C8 are listed in Table 2. The steels in Comparative Examples 5 to 8 exhibited UTS levels greater than 1,100 MPa. Comparative Examples 9 to 11

[0068] Cold-rolled steel sheets with compositions as listed in Table 2, Samples No. C9 to C11, were subjected to a two-stage annealing process similar to that illustrated in Fig. 1, except that the soaking or holding temperature in the second annealing was outside the preferred ranges of the invention. The mechanical properties of Samples No. C9 to C11 are listed in Table 2. Comparative example 12

[0069] A cold-rolled sheet with a composition listed in Table 2, Sample No. C12, was subjected to a two-stage annealing process similar to that illustrated in Fig. 2, except that the temperature of the retention zone in the second annealing was outside the preferred range of the invention. The mechanical properties of Sample No. C12 are listed in Table 2. Petition 870260020852, dated 06 / 03 / 2026, page 29 / 65 / 24 Table 2 Sample No. Product Type Process C Mn Si Other YS (MPa) UTS (MPa) TE (%) UTS^TE Lambda 1 Super elongation of CR 780 GEN3 CAL + CAL 0.22 1.5 1.5 657 831 39.2 32575 44 2 CR 980 GEN3 CAL + CAL 0.185 2.2 1.4 630 1030 25 25750 30 3 980 GEN3 hot dipped CAL + CGL 0.22 2.25 0.45 0.8 of Al, 0.02 of Ti, 0.02 of Nb 576 988 30.2 29838 20 4 980 GEN3 hot dipped Unique process in newly designed CGL 0.185 2.2 1.4 580 998 29.1 29042 5 Super elongation of CR 980 GEN3 CAL + CAL 0.35 1.5 1.5 685 999 38.2 38161 25 6 CR 780 GEN3 CAL + CAL 0.175 1.8 1.5 630 840 33 27720 45 7 780 GEN3 hot-dip CAL + CGL 0.2 2.3 0.4 1.0 Al 533 915 32.8 30012 30 8 780 GEN3 hot-dip Unique process in newly designed CGL 0.2 2.3 0.4 1.0 Al 589 865 34.4 29756 Petition 870260020852, dated 06 / 03 / 2026, page 30 / 65 / 24 Table 2 (continued) Sample No. Product Type Process C Mn Si Other YS (MPa) UTS (MPa) TE (%) UTS^TE Lambda C1 Alloy 1 CAL + CAL 0.105 1.55 1.2 512 666 32.6 21712 67 C2 Alloy 2 CAL + CAL 0.14 1.5 1.3 556 690 30.2 20838 59 C3 Alloy 3 CAL + CAL 0.17 1.1 1.1 560 686 26.9 18453 53 C4 Alloy 4 CAL + CAL 0.13 0.9 0.9 533 618 26.0 16068 81 C5 Alloy 1 CAL + CAL 0.21 2.15 1.5 0.003 of B 597 1125 17.2 19350 25 C6 Alloy 2 CAL + CAL 0.2 2.2 1.5 0.2 of Mo: 585 1148 16.3 18712 13 C7 Alloy 3 CAL + CAL 0.23 2.4 1.5 0.003 of B 683 1231 16.2 19942 6.7 C8 Alloy 4 CAL + CAL 0.19 2.64 2.0 635 1359 14.8 20113 3.4 C9 Alloy 1 - High Soaking (849 C) CAL + CAL 0.18 2.2 1.34 693 1058 18.2 19256 25 C10 Alloy 2 - Low Retention (350°C) CAL + CAL 0.18 2.2 1.34 602 1035 21.2 21942 30 C11 Alloy 3 - High Retention (450°C) CAL + CAL 0.18 2.2 1.34 477 1059 19.7 20862 19 C12 Alloy 4 - High Retention (471°C) CAL + CGL 0.22 2.4 0.4 0.8 of Al 465 1012 23.0 23276 16.5 Petition 870260020852, dated 06 / 03 / 2026, page 31 / 65 / 24

[0070] Fig. 15 represents the total elongation (TE) and tensile strength limit (UTS) of Samples 1 to 8 of Examples 1 to 8, as well as Samples C1 to C12 of Comparative Examples C1 to C12. A line corresponding to UTS^TE of 25,000 is roughly drawn in Fig. 15. As can be seen, the high-strength steel sheet samples produced according to the present invention have superior combinations of strength and elongation compared to the comparative samples, i.e., high total elongation properties at high UTS levels are observed for the examples of the invention. The steels of Sample 1 to Sample 8 fall into the category of Generation 3 advanced high-strength steels, which are highly desirable for the automotive and other industries. Example 9

[0071] Grinding tests were conducted on samples labeled M1 to M5 in Table 3 below using the CAL+CAL or CAL+CGL process. For Samples M1, M2, and M5, the CAL+CAL processing times and temperatures shown in Fig. 1 were used. For Samples M3 and M4, the CAL+CGL processing times and temperatures shown in Fig. 2 were used. Table 3 Grinding test results Grinding Test No. Process C Mn Si Others Coating YS (MPa) UTS (MPa) TE (%) UTS-TE Lambda M1 CAL + CAL 0.22 1.4 1.4 None 627 810 38.6 31266 61 M2 CAL + CAL 0.185 2.2 1.4 None 624 1009 25 25255 38 M3 CAL + CGL 0.23 2.4 0.4 0.8 Al, 0.02 Ti, 0.02 Nb HDGI 567 989 26.4 26110 24 M4 CAL + CGL 0.22 2.3 0.4 0.8 Al, 0.02 Ti, 0.02 Nb HDGI 655 941 30.9 29077 33 M5 CAL + CAL 0.19 2.25 1.5 None 635 1048 25.3 26514 29 Petition 870260020852, dated 06 / 03 / 2026, page 32 / 65 / 24

[0072] Fig. 16 shows the strength-elongation balance of the crush test materials, all meeting a minimum UTS^TE of 25,000. The test materials exhibited lambda values ​​greater than 20%. Example 10

[0073] Cold-rolled and hot-rolled steel sheets with a composition of 0.23 wt percent C, 2.3 wt percent Mn, 0.6 wt percent Si, and 0.8 wt percent Al corresponding to Samples 9A to 12B in Table 4 were subjected to a two-stage annealing process as shown in Fig. 17. In Table 4, cold-rolled samples are listed as substrate types “CR” and hot-rolled samples are listed as substrate types “HR”. The mechanical properties of Samples No. 9A to 12B are listed in Table 4. The hot-rolled substrate samples showed exceptional YS, UTS, TE, and hole expansion properties comparable to the cold-rolled samples, demonstrating that a hot-rolled substrate processed directly in the two-stage annealing process can produce Generation 3 AHSS properties. Furthermore, as indicated in the EBSD phase maps shown in Figs.Figures 18 and 19, in which the retained austenite grains are lighter than the ferrite grains, show similar austenite contents, distributions, and morphologies for the hot-rolled material when compared to the cold-rolled material. Fig. 18 shows the austenite content of cold-rolled sample 11A, and Fig. 19 shows the austenite content of hot-rolled sample 12A. A fine, predominantly equiaxed distribution of austenite is observed in both microstructures. Table 4 Specimen ID Substrate Type Cycle Type YS (MPa) UTS (MPa) TE (%) UTS-TE Hole Expansion Ratio (%) 9A CR 427°C; (800°F) Retention - GI 706.32 973.1 31.39 30545.61 35 9B 712.02 959.2 34.41 33006.07 Petition 870260020852, dated 06 / 03 / 2026, page 33 / 65 / 24 10A UR 427°C; (800°F) Retention - GI 696.37 963.8 30.26 29164.59 46 10B 695.40 954.4 35.18 33575.79 11A CR 443°C; (830°F) Retention - GI 618.23 1032.9 - - 31 11B 614.00 1030.4 28.04 28892.42 12A HR 443°C; (830°F) Retention - GI 650.39 1002.2 27.21 27269.86 35 12B 622.82 1007.6 27.75 27960.90

[0074] As used in this document, “including,” “containing,” and similar terms are understood in the context of this application as synonymous with “comprising” and are therefore open-ended and do not exclude the presence of elements, materials, phases, or steps of the method. As used in this document, “consisting of” is understood in the context of this application as excluding the presence of any unspecified element, material, phase, or step of the method. As used in this document, “essentially consisting of” is understood in the context of this application to include the specified elements, materials, phases, or steps of the method, where applicable, and also to include any unspecified elements, materials, phases, or steps of the method that do not materially affect the basic or novel features of the invention.

[0075] Although the numerical ranges and parameters that define the broad scope of the invention are approximations, the numerical values ​​presented in the specific examples are reported as accurately as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in its respective test measurements.

[0076] It should be understood that any numerical range cited here is intended to include all subranges included herein. For example, a range of “1 to 10” is intended to include all subranges between (and including) the minimum recited value of 1 and the maximum recited value of 10, i.e., having a minimum value equal to or greater than 1 and a maximum value equal to or less than 10. Petition 870260020852, dated 06 / 03 / 2026, p. 34 / 65 / 24

[0077] In this application, the use of the singular includes the plural and the plural encompasses the singular, unless specifically indicated otherwise. Furthermore, in this application, the use of “or” means “and / or” unless specifically indicated otherwise, even though “and / or” may be used explicitly in certain cases. In this application and the attached claims, the articles “a”, “an”, and “the” include plural referents, unless expressly and unequivocally limited to one referent.

[0078] Considering that particular embodiments of this invention have been described above for illustrative purposes, it will be evident to those skilled in the art that numerous variations of the details of the present invention can be made without departing from the invention. Petition 870260020852, dated 06 / 03 / 2026, page 35 / 65

Claims

1 / 2 CLAIMS 1. High-strength hot-rolled steel sheet product, characterized in that it comprises 0.12 to 0.5 percent by weight of C, 1 to 3 percent by weight of Mn, 0.8 to 3 percent by weight of a combination of Si and Al, and a balance of Fe, wherein the steel sheet product has been subjected to a two-stage annealing process, comprises equiaxed retained ferrite and austenite grains with an average aspect ratio of less than 2:1, and less than 15 percent by volume of initial untempered martensite is free of tempered martensite, and has a combined tensile strength and total elongation UTS *TE greater than 25,000 MPa% and a hole expansion ratio greater than 35 percent.

2. High-strength hot-rolled steel sheet product according to claim 1, characterized in that Si comprises up to 2 percent by weight, Al comprises up to 2 percent by weight, and the hot-rolled steel sheet product further comprises up to 0.05 percent by weight of Ti and up to 0.05 percent by weight of Nb.

3. High-strength hot-rolled steel sheet product according to claim 2, characterized in that C comprises from 0.15 to 0.4 percent by weight, Mn comprises from 1.3 to 2.5 percent by weight, Si comprises from 0.2 to 1.8 percent by weight, Al comprises up to 1.5 percent by weight, Ti comprises up to 0.03 percent by weight and Nb comprises up to 0.03 percent by weight.

4. High-strength hot-rolled steel sheet product according to claim 1, characterized in that ferrite comprises at least 50 percent by volume, and retained austenite comprises 5 to 25 percent by volume.

5. High-strength hot-rolled steel sheet product according to claim 1, characterized in that the retained austenite has an average grain size of less than 10 microns.

6. High-strength hot-rolled steel sheet product according to claim 1, characterized in that the retained austenite has an average grain size of less than 1 micron.

7. High-strength hot-rolled steel sheet product according to claim 1, characterized in that the hot-rolled steel sheet product has a tensile strength of 720 to 1,100 MPa and has a total elongation of at least 20 percent.

8. High-strength hot-rolled steel sheet product according to claim 1, characterized in that the UTS-TE is at least 27,000 MPa%.

9. High-strength hot-rolled steel sheet product according to claim 1, characterized in that it additionally comprises a zinc-based coating on the hot-rolled steel sheet product.

10. High-strength hot-rolled steel sheet product according to claim 1, characterized in that the retained austenite has an average grain size of less than 0.5 microns. Petition 870260020852, dated 06 / 03 / 2026, p. 37 / 65

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