High strength steel products and annealing process for producing same

Abstract

The present invention involves subjecting a steel plate product having a controlled composition to a two-step annealing process to produce a plate product with a desired microstructure and good mechanical properties, such as high strength and ultra-high formability. The steel plate product can be cold-rolled or hot-rolled. Steel processed according to the present invention has a good combination of ultimate tensile strength and total elongation (UTS·TE), falling into the category of third-generation advanced high-strength steels, which are highly desired by various industries, including automotive manufacturers.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application 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, and is a continuation-in-part of U.S. Patent Application No. 15 / 591,344 (now U.S. Patent No. 10,385,419), filed May 10, 2017. This application is also a continuation-in-part of U.S. Patent Application No. 15 / 591,344, which is a continuation-in-part of U.S. Patent Application No. 16 / 459,757, filed July 2, 2019. All of the foregoing applications are incorporated herein by reference.

[0002] FIELD OF THE INVENTION The present invention relates to high strength steel products having good properties and to an annealing process for producing the products. [Background technology]

[0003] <Background information> Over the past few years, the global steel industry has focused on developing third-generation advanced high-strength steels (AHSS) for the automotive market. These Generation 3 steels offer a good balance of tensile strength and elongation, typically with a UTS·TE range of approximately 20,000 MPa% or greater. However, the steel industry has struggled to commercialize third-generation AHSS. This is because many of these steels require high alloying elements—typically greater than 4% manganese by weight—making them difficult to manufacture using conventional steelmaking equipment. Furthermore, currently commercially available AHSS steels are difficult to weld using techniques such as spot welding, difficult to coat with zinc-based galvanic coatings, and difficult to fabricate into the thin-gauge plates required for a wide range of applications. Summary of the Invention

[0004] The present invention provides a steel plate product having a controlled composition and subjected to a two-step annealing process to produce a plate product with a desired microstructure and good mechanical properties, such as high strength and ultra-high formability. The steel plate product may be cold-rolled or hot-rolled. Steel processed according to the present invention has a combined ultimate tensile strength and total elongation (UTS·TE) property of greater than 25,000 MPa% when tested using standard subsize ASTM or full-size JIS tensile test procedures. Furthermore, steel produced according to the present invention has a good combination of TE and hole expansion, and both global and local formability are good. Steels with these properties fall into the category of third-generation advanced high-strength steels and are highly desired by various industries, including automotive manufacturers.

[0005] One aspect of the present invention is to provide a high strength rolled steel plate product comprising 0.12-0.5 wt% C, 1-3 wt% Mn, and 0.8-3 wt% Si and Al in combination, the steel plate product being subjected to a two-step annealing process and comprising ferrite and substantially equiaxed retained austenite having an average aspect ratio of less than 3:1, and having a combined ultimate tensile strength and total elongation, UTS·TE, greater than 25,000 MPa%.

[0006] Another aspect of the present invention is to provide a method for producing a high-strength rolled steel plate product containing 0.12-0.5 wt% C, 1-3 wt% Mn, and 0.8-3 wt% Si and Al in combination, which comprises subjecting the steel plate product to a first-step annealing process to obtain a mainly martensite structure, and then subjecting the steel plate product to a second-step process, which comprises soaking the steel plate product in a transformation interval at a temperature of 720-850°C, and then holding the steel plate product at a temperature of 370-445°C.

[0007] These and other aspects of the present invention will become more apparent from the following description. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 shows a two-step annealing process plotting temperature against time according to one embodiment of the present invention.

[0009] [Figure 2] FIG. 2 shows a two step annealing process according to another embodiment of the present invention, plotting temperature against time.

[0010] [Figure 3] FIG. 3 shows a two-step annealing process in which the two-step heating process is optionally combined with a molten zinc-based coating process in one manufacturing facility, plotting temperature against time.

[0011] [Figure 4] FIG. 4 is a plot of temperature versus time for a two-step annealing process comprising a soak zone and a hold zone in a thermal cycle according to one embodiment of the present invention.

[0012] [Figure 5] FIG. 5 is an electron backscatter diffraction (EBSD) micrograph showing the microstructure of a high strength steel plate product in accordance with one embodiment of the present invention. [Figure 6] FIG. 6 is an electron backscatter diffraction (EBSD) micrograph showing the microstructure of a high strength steel plate product in one embodiment of the present invention.

[0013] [Figure 7] FIG. 7 is an optical micrograph of the heat treated steel sheet product shown in FIG. 1, showing dark ferrite grains and light austenite grains.

[0014] [Figure 8] 8 is a bar graph showing the aspect ratios of the austenite grains shown in FIG. 7.

[0015] [Figure 9] FIG. 9 is a graph of a high strength steel plate product in accordance with one embodiment of the present invention showing the austenite grain size distribution. [Figure 10] FIG. 10 is a graph of a high strength steel sheet product according to one embodiment of the present invention showing the ferrite grain size distribution.

[0016] [Figure 11] FIG. 11 is an EBSD micrograph showing the microstructure of a high strength steel plate product produced by the process shown in FIG.

[0017] [Figure 12] FIG. 12 is an EBSD micrograph showing a steel sheet product made by the process shown in FIG. [Figure 13] FIG. 13 is an EBSD micrograph showing a steel sheet product made by the process shown in FIG.

[0018] [Figure 14] FIG. 14 is an EBSD micrograph of a steel plate product produced by the process shown in FIG.

[0019] [Figure 15] FIG. 15 is a graph showing the relationship between total elongation and ultimate tensile strength for a high strength steel plate product of the present invention compared to other steel plate products made by processes outside the scope of the present invention.

[0020] [Figure 16] FIG. 16 is a graph showing the relationship between total elongation and ultimate tensile strength for high strength steel products produced in a mill test in accordance with an embodiment of the present invention.

[0021] [Figure 17] FIG. 17 is a plot of temperature versus time for cold rolled and hot rolled substrates subjected to a thermal cycle according to one embodiment of the present invention.

[0022] [Figure 18] FIG. 18 is an EBSD micrograph of the heat treated cold rolled substrate shown in FIG. 17, showing dark ferrite grains and light retained austenite grains.

[0023] [Figure 19] FIG. 19 is an EBSD micrograph of the heat treated hot rolled substrate shown in FIG. 17, showing dark ferrite grains and light retained austenite grains. DETAILED DESCRIPTION OF THE INVENTION

[0024] <Detailed explanation> The high-strength steel plate products of the present invention have a controlled composition that, in combination with a controlled annealing process, produces a desired microstructure and favorable mechanical properties, including high strength and ultra-high formability. In certain embodiments, the steel composition can include carbon, manganese, and silicon, along with any other suitable alloying elements known to those skilled in the art. Examples of steel compositions containing ranges of C, Mn, Si, Al, Ti, and Nb are shown in Table 1 below. [Table 1]

[0025] In addition to the amounts of C, Mn, Si, Al, Ti, and Nb listed in Table 1, the steel composition may contain other elements in small amounts or at impurity levels, such as, for example, up to 0.015 S, up to 0.03 P, up to 0.2 Cu, up to 0.02 Ni, up to 0.2 Cr, up to 0.2 Mo, up to 0.1 Sn, up to 0.015 N, up to 0.1 V, and up to 0.004 B. As used herein, the term "substantially free," in reference to the composition of a steel plate product, means that the particular element or material is not intentionally added and is only present as an impurity or in trace amounts.

[0026] In the steel plate product of the present invention, C increases strength and promotes the formation of retained austenite. Mn brings about hardening and acts as a solid solution strengthener. Si suppresses the precipitation of iron carbide during heat treatment and increases the amount of retained austenite. Al suppresses the precipitation of iron carbide during heat treatment and increases the amount of retained austenite. Ti and Nb can act as grain refiners that improve strength.

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

[0028] Steel sheet products having the above composition are subjected to a two-stage annealing process, which is described in more detail below. The resulting steel sheet products have been found to have favorable mechanical properties, including favorable ultimate tensile strength, high elongation, high lambda values, high bendability, and high yield to tensile ratio (YS / UTS).

[0029] In certain embodiments, the steel sheet product has an ultimate tensile strength (UTS) of 700 to 1,100 MPa or more. In certain embodiments, the steel sheet product has an ultimate tensile strength of more than 700 MPa, for example, 720 to 1,100 MPa, or 750 to 1,050 MPa.

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

[0031] The steel plate products typically have a lambda (λ) value measured by a standard hole expansion test of greater than 20%, such as greater than 25%, or greater than 30%, or greater than 35%. The total expansion ratio or lambda may be greater than 20%, such as from 22 to 80%, or from 25 to 60%.

[0032] In certain embodiments, high values of both total elongation (TE) and hole expansion (λ) result in a steel sheet product with good global and local formability.

[0033] In the steel sheet products of the present invention, a strength-elongation balance (UTS·TE) of greater than 25,000 is observed, which corresponds to the third generation steel grades highly sought after by industries such as the automotive industry. In certain embodiments, the UTS·TE value is greater than 27,000, or greater than 30,000, or greater than 35,000.

[0034] According to certain embodiments of the present invention, the final microstructure of the steel sheet product is predominantly ferrite with a minor amount of retained austenite and a small amount of fresh martensite, e.g., at least 50% and up to 80% or more ferrite, retained austenite, e.g., 5-25%, and fresh martensite, e.g., 0-10% 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 herein, the volume percent of retained austenite is determined by EBSD techniques.

[0035] In certain embodiments, the retained austenite comprises 1 to 25 volume %, for example 5 to 20 volume %. The amount of fresh martensite can be less than 15 volume %, or less than 10 volume %, or less than 5 volume %. In certain embodiments, the steel sheet product is substantially free of fresh martensite. It has been found that when the amount of fresh martensite exceeds 15%, the hole expansion value significantly decreases, e.g., local formability significantly decreases.

[0036] At least a portion of the ferrite is formed by recrystallization and / or tempering of martensite during the heating section, as described below, or by decomposition of austenite during the cooling and holding section of the second annealing process. Some of the ferrite is considered bainitic ferrite. The ferrite, austenite, and martensite phases are fine-grained, e.g., have an average grain size of less than 10 microns, e.g., less than 5 microns, or less than 3 microns. For example, the ferrite grain size may be in the range of less than 10 microns, e.g., less than 8 microns, or less than 6 microns. The austenite grain size may be in the range of less than 2 microns, e.g., less than 1 micron, or less than 0.5 microns. If martensite is present, the martensite grain size may be in the range of less than 10 microns, e.g., less than 8 microns, or less than 6 microns.

[0037] The austenite grains are substantially equiaxed, e.g., have an average aspect ratio of less than 3:1, or even less than 2:1, e.g., about 1:1. It has been found that amounts of retained austenite below about 5% significantly reduce total elongation (TE). It has also been found that amounts of retained austenite greater than 25% can only be achieved with very high carbon contents, which results in poor weldability.

[0038] In a specific embodiment of the present invention, a two-step annealing process is used to produce advanced high-strength steel products with the above-mentioned excellent mechanical properties. Multiple methods for performing heat treatment can be used in each of the first and second annealing steps. Examples of the two-step annealing process are shown in Figures 1-3 and are described below. Figure 1 shows a continuous annealing line (CAL) followed by a continuous annealing line (CAL). Figure 2 shows a CAL + continuous galvanizing line (CGL) manufacturing route. Figure 3 shows a line specially designed to perform both CAL + CAL or CAL + CGL processes within a single facility. While Figure 3 shows an embodiment of a direct-fired furnace (DFF) followed by a radiant tube (RT) furnace, other embodiments, such as all radiant tubes or electric radiant heating, can also be used to achieve the desired thermal cycle.

[0039] <Step 1> The purpose of the first step of the annealing process is to produce a martensitic microstructure in a cold-rolled or hot-rolled steel sheet product. In the first annealing stage of the first step, an annealing temperature above the A3 temperature is typically used, e.g., 820°C or higher. In certain embodiments, the annealing temperature in the first stage may typically be in the range of 830-980°C, e.g., 830-940°C, 840-930°C, or 860-925°C. In certain embodiments, the peak annealing temperature may typically be held for 20 seconds or longer, and the holding time may be, e.g., 20-500 seconds, or 30-200 seconds. Heating may be performed by conventional techniques, such as a non-oxidizing or oxidizing direct-fired furnace (DFF), oxygen-enriched DFI, induction heating, gas radiant tube heating, or electric radiant heating. Examples of heating systems suitable for use in the process of the present invention are described in U.S. Patent Nos. 5,798,007, 7,368,689, 8,425,225, and 8,845,324, all assigned to Fives Stein, U.S. Patent Application Publication No. 2009 / 0158975, and International Publication No. WO2015 / 083047. Other examples of heating systems suitable for use in the process of the present invention include U.S. Patent No. 7,384,489, all assigned to Drever International, and U.S. Patent No. 9,096,918, all assigned to Nippon Steel & Sumitomo Metal Corporation. Other known suitable heating systems and processes can be used in Steps 1 and 2.

[0040] In the first stage, after the peak annealing temperature has been reached and held for a predetermined time, the cold-rolled or hot-rolled steel sheet is quenched to room temperature or to a controlled temperature above room temperature, as described in detail below. The quenching temperature does not necessarily have to be room temperature to form a predominantly martensite microstructure, but should be above the martensitic transformation start temperature (M S ), and preferably, the temperature should be lower than the martensitic transformation finish temperature (M FIn certain embodiments, the steel sheet product may be cooled to a temperature below 300°C, for example below 200°C, between the first and second step processes.

[0041] Quenching can be accomplished by conventional techniques such as water quenching, submerged knife / nozzle water quenching, gas quenching, rapid quenching using a combination of cold water, hot water, or hot water and gas, aqueous solution quenching, other liquid or gas fluid quenching, chilled roll quenching, water mist spray, wet flash quenching, non-oxidizing wet flash quenching, etc. Quenching rates of 30 to 2000°C / sec are typically used.

[0042] Various types of cooling and quenching systems and processes known to those skilled in the art can be adapted for use in the process of the present invention. Suitable cooling / quenching systems and processes conventionally used on a commercial basis include water quenching, water mist cooling, dry flash, wet flash, oxidative cooling, non-oxidative cooling, alkane fluid-to-gas phase conversion cooling, hot water quenching, two-step water quenching, roll quenching, high-percentage hydrogen or helium gas jet cooling, etc. For example, the dry flash and / or wet flash oxidative and non-oxidative cooling / quenching disclosed in Fives Stein's International Publication No. WO 2015 / 083047 can be used. Other Fives Stein patents describing cooling / quenching systems and processes configured for use in the process of the present invention include U.S. Patent Nos. 6,464,808 B2, 6,547,898 B2, and 8,918,199 B2, and U.S. Patent Application Publication Nos. US 2009 / 0158975 A1, 2009 / 0315228 A1, and 2011 / 0266725 A1. Other examples of cooling / quenching systems and processes configured for use in the process of the present invention include U.S. Patent Nos. 8,359,894 B2, 8,844,462 B2, and 7,384,489 B2, and U.S. Patent Application Publication Nos. 2002 / 0017747 A1 and 2014 / 0083572 A1.

[0043] In certain embodiments, after the first stage peak annealing temperature is reached and the steel is quenched to form martensite, the martensite is optionally tempered to soften the steel somewhat and make it easier to further process. Tempering is accomplished by raising the temperature of the steel from room temperature to about 500°C and holding for up to 600 seconds. If tempering is utilized, the tempering temperature may be held constant or may be varied within this preferred range.

[0044] After tempering, the temperature is ramped down to room temperature. The ramp-down rate may typically be in the range of 1-40°C / sec, e.g., 2-20°C / sec. Tempering is not required for single-pass furnaces, as shown in Figure 3.

[0045] <Step 2> The second step of the annealing process involves a first stage conducted at a relatively high annealing temperature and a second stage conducted at a relatively low temperature. These stages are configured as the second annealing "soaking" and "holding" zones, as depicted in Figure 4. The temperatures are controlled to promote the development of the desired microstructure in the final product.

[0046] In the first annealing stage of the second step, the temperature of the soaking zone can be between A1 and A3, for example, an annealing temperature of 720°C or higher. In certain embodiments, the temperature of the soaking zone can typically be in the range of 720 to 850°C, for example, 760 to 825°C. In certain embodiments, the peak annealing temperature can typically be held for at least 15 seconds, with a holding time of, for example, 20 to 300 seconds, or 30 to 150 seconds.

[0047] During the first stage of the second step, the temperature of the soaking zone can be relatively low below Ms, e.g., room temperature, by heating the steel at an average rate of 0.5-50°C / sec, e.g., about 2-20°C / sec. In certain embodiments, the ramp-up takes 25-800 seconds, e.g., 100-500 seconds. Heating in the first stage of the second step can be accomplished by any suitable heating system or process, such as radiant heating, induction heating, heating in a direct-fired furnace, etc.

[0048] After reaching the soaking zone temperature and holding for a predetermined time, the steel is cooled to a holding zone at a controlled temperature above room temperature. In certain embodiments, the steel sheet product is maintained at a temperature above 300°C between the second soaking process and the second holding process. Cooling from the soaking zone to the holding zone can be accomplished by conventional techniques, such as water cooling or gas cooling. Average cooling rates of 5 to 400°C / sec are typically used. Any suitable type of cooling and quenching system, including those described above, can be used to cool from the soaking temperature to the holding temperature.

[0049] In an embodiment of the present invention, the holding zone step is typically carried out at a temperature of 360 to 445° C., for example, a temperature of 370 to 440° C. The holding time in the holding zone may be up to 800 seconds, for example, 30 to 600 seconds.

[0050] The temperature of the holding zone may be kept constant or may fluctuate somewhat within the preferred temperature range. If the steel is to be hot-dip coated after the holding step, the steel may be reheated by induction heating or other heating methods and placed in a hot-dip coating pot at the appropriate temperature for good coating results.

[0051] In certain embodiments, after the temperature of the holding zone is maintained for a desired period of time, the temperature can be ramped down to room temperature. Such ramping down may typically take 10 to 1000 seconds, for example, about 20 to 500 seconds. The rate of such ramping down may typically be in the range of 1 to 1000°C / sec, for example, 2 to 20°C / sec.

[0052] According to certain embodiments, one or both of the first and second annealing processes can be performed in a continuous annealing line (CAL). After the 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 holding zone. Galvanizing temperatures can typically range from 440 to 480°C, e.g., 450 to 470°C. In certain embodiments, the galvanizing step can be performed as part of a second-stage annealing process on a continuous galvanizing line (CGL), such as that shown in FIG. 2. This CAL+CGL process can be used to produce hot-dip galvanized products based on zinc or zinc alloys, which can be reheated after coating to produce iron-zinc galvannealed-type coated products. An optional nickel-based coating step can be performed between the CAL and CGL steps to improve the zinc coating properties. Using a second-stage continuous galvanizing line improves production efficiency in the production of GEN3 (Generation 3) coated products compared to using the CAL+CAL+EG route.

[0054] Galvanized or hot-dip coated products with zinc-based alloys can also be produced in specially designed CGLs that allow two-step annealing in a single line, as shown in Figure 3. In this case, galvannealing may be optional. Additionally, a single manufacturing facility can be specially designed and constructed to combine the two-step thermal processes to produce the uncoated Generation 3 steels defined in this invention. [Example]

[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.

[0056] Example 1 A cold-rolled steel sheet having the composition of Sample No. 1 in Table 2 was subjected to the two-step annealing process shown in Figure 1. The microstructure of the resulting product is shown in Figures 5 and 6. Figure 5 shows dark ferrite grains and light austenite grains, as determined by EBSD technique using commercially available EDAX orientation image microscope software.

[0057] <Example 2> A cold-rolled steel sheet having the composition of Sample No. 2 in Table 2 was subjected to the two-step annealing process shown in Figure 1. The microstructure of the resulting product is shown in Figure 11. The mechanical properties of Sample No. 2 are listed in Table 2. The austenite grain size distribution is shown in Figure 9, and the ferrite grain size distribution is shown in Figure 10. The average austenite grain size is less than 1 micron, and the average ferrite grain size is less than 10 microns.

[0058] The microstructure includes about 80 volume percent ferrite having an average grain size of about 5 microns, about 10 volume percent substantially equiaxed retained austenite having an average grain size of about 0.5 microns, and about 10 volume percent fresh martensite having an average grain size of about 5 microns. The mechanical properties of Sample No. 1 are listed in Table 2.

[0059] Example 3 A cold-rolled steel sheet having the composition of Sample No. 3 in Table 2 was subjected to the two-step annealing process shown in Figure 2. The microstructure of the resulting product is shown in Figures 12 and 13. In Figure 13, austenite is light in color and ferrite is dark in color. The mechanical properties of Sample No. 3 are listed in Table 2.

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

[0061] <Example 5> A cold-rolled steel sheet having the composition of Sample No. 5 in Table 2 was subjected to the two-step annealing process shown in Figure 1. The mechanical properties of Sample No. 5 are listed in Table 2.

[0062] Example 6 A cold-rolled steel sheet having the composition of Sample No. 6 in Table 2 was subjected to the two-step annealing process shown in Figure 1. The mechanical properties of Sample No. 6 are listed in Table 2. Figure 7 is an optical image showing the microstructure of a steel having the composition of Sample No. 6 in Table 2, which was subjected to the two-step annealing process shown in Figure 1. In Figure 7, the dark areas in the image are ferrite grains, and the light areas are austenite grains. Figure 8 is a graph showing the aspect ratio of the austenite grains shown in Figure 7. The optical image in Figure 7 was used to determine the aspect ratio of the austenite grains using image analysis software obtained commercially. Figure 7 shows that the average aspect ratio of the austenite grains is less than 3:1.

[0063] Example 7 A cold-rolled steel sheet having the composition of Sample No. 7 in Table 2 was subjected to the two-step annealing process shown in Figure 2. The mechanical properties of Sample No. 7 are listed in Table 2.

[0064] Example 8 A cold-rolled steel sheet having the composition of Sample No. 8 in Table 2 was subjected to the two-step annealing process shown in Figure 3. The mechanical properties of Sample No. 8 are listed in Table 2.

[0065] The steels of Examples 1 to 8 exhibited UTS levels in the range of 700 to 1,100 MPa.

[0066] <Comparative Examples 1 to 4> Cold-rolled steel sheets having the compositions of Samples No. C1 to C4 in Table 2 were subjected to the two-step annealing process shown in Figure 1. The mechanical properties of Samples No. C1 to C4 are listed in Table 2. The steels of Comparative Examples 1 to 4 exhibited UTS levels of less than 700 MPa.

[0067] <Comparative Examples 5 to 8> Cold-rolled steel sheets having the compositions of Samples No. C5 to C8 in Table 2 were subjected to the two-step annealing process shown in Figure 1. The mechanical properties of Samples No. C5 to C8 are listed in Table 2. The steels of Comparative Examples 1 to 4 exhibited UTS levels exceeding 1,100 MPa.

[0068] <Comparative Examples 9 to 11> Cold-rolled steel sheets having the compositions of Samples C9 to C11 in Table 2 were subjected to an annealing process similar to the two-step annealing process shown in Figure 1. Note that the soaking or holding temperature of the second annealing step was outside the preferred range of the present invention. The mechanical properties of Samples C9 to C11 are listed in Table 2.

[0069] <Comparative Example 12> A cold-rolled steel sheet having the composition of Sample No. C12 in Table 2 was subjected to an annealing process similar to the two-step annealing process shown in Figure 2. Note that the holding temperature of the second annealing step was outside the preferred range of the present invention. The mechanical properties of Sample No. C12 are listed in Table 2. [Table 2-1] [Table 2-2]

[0070] Figure 15 plots total elongation (TE) versus ultimate tensile strength (UTS) for Samples 1-8 of Examples 1-8 and Samples C1-C12 of Comparative Examples 1-12. A line corresponding to 25,000 UTS·TE is outlined in Figure 15. As can be seen from this figure, the high-strength steel sheet samples manufactured according to the present invention exhibit a superior combination of strength and elongation compared to the comparative samples. That is, the examples of the present invention are observed to have high total elongation at high UTS levels. The steels of Samples 1-8 fall into the category of third-generation advanced high-strength steels, which are highly desirable for the automotive and other industries.

[0071] Example 9 Mill trials were performed on samples designated M1-M5 in Table 3 below using either the CAL+CAL process or the CAL+CGL process. For samples M1, M2, and M5, the CAL+CAL treatment times and temperatures shown in Figure 1 were used. For samples M3 and M4, the CAL+CGL treatment times and temperatures shown in Figure 2 were used. [Table 3]

[0072] Figure 16 shows the strength-elongation balance of the mill-tested materials, all of which met the minimum UTS·TE of 25,000. The lambda values of the test materials were greater than 20%.

[0073] Example 10 Samples No. 9A to No. 12B in Table 4 are cold-rolled and hot-rolled steel sheets with compositions of 0.23 wt% C, 2.3 wt% Mn, 0.6 wt% Si, and 0.8 wt% Al. These steel sheets were subjected to the two-stage annealing process shown in Figure 17. In Table 4, the cold-rolled samples are designated as the "CR" substrate type, and the hot-rolled samples are designated as the "HR" substrate type. The mechanical properties of Samples No. 9A to No. 12B are shown in Table 4. The hot-rolled substrate samples exhibited excellent YS, UTS, TE, and hole expansion properties comparable to those of the cold-rolled samples. This indicates that the hot-rolled substrate directly subjected to the two-stage annealing process can achieve third-generation AHSS properties. Furthermore, the austenite content, distribution, and morphology were observed in the hot-rolled material similar to that in the cold-rolled material, as shown in the EBSD phase maps shown in Figures 18 and 19, where the retained austenite grains appear brighter than the ferrite grains. Figure 18 shows the austenite content of cold-rolled sample 11A, and Figure 19 shows the austenite content of hot-rolled sample 12A. In both microstructures, the austenite is observed to be fine and predominantly equiaxed. [Table 4]

[0074] As used herein, terms such as "including," "comprising," "containing," and the like are open-ended terms and are understood to not exclude the presence of additional elements, materials, phases, or process steps not recited in this application. As used herein, the term "consisting of" is understood to exclude the presence of any unspecified element, material, phase, or process step. As used herein, the term "consisting essentially of" is understood to include the specified elements, materials, phases, or process steps, where applicable, and to include any unspecified elements, materials, phases, or process steps that do not materially affect the basic or novel characteristics of the invention.

[0075] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible, however, any numerical value inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0076] It should also be understood that all numerical ranges set forth herein are intended to include all subranges encompassed therein. For example, a range of "1" to "10" is intended to include the subrange between the minimum value of 1 and the maximum value of 10, where the minimum value is 1 or greater than 1 and the maximum value is 10 or less than 10.

[0077] In this application, the use of the singular includes the plural and the plural includes the singular, unless expressly stated otherwise. Also, in this application, the use of "or" means "and / or" unless expressly stated otherwise, even if the word "and / or" is explicitly used in a particular embodiment. In this specification and claims, the articles "a," "an," and "the" include plural referents unless expressly and unambiguously limited to one referent.

[0078] While specific embodiments of the present invention have been described above for purposes of illustration, it will be apparent to those skilled in the art that many changes in the details of the invention can be made without departing from the invention.

Claims

1. A high-strength hot-rolled steel plate product comprising a steel containing 0.12 to 0.5 wt% C, 1 to 3 wt% Mn, 0.8 to 3 wt% of a combination of Si and Al, and the balance being Fe and unavoidable impurities, The hot rolled steel plate product has been subjected to a two-step annealing process to produce a high strength hot rolled steel plate product comprising ferrite grains and substantially equiaxed retained austenite grains having an average aspect ratio of less than 2:1, and having a combined ultimate tensile strength and total elongation, UTS·TE, greater than 25,000 MPa% and a hole expansion ratio greater than 35%.

2. 2. The high strength hot rolled steel sheet product of claim 1, wherein Si comprises up to 2 wt.% and Al comprises up to 2 wt.%, and said hot rolled steel sheet product further comprises Ti up to 0.05 wt.% and Nb up to 0.05 wt.%.

3. 3. The high strength hot rolled steel sheet product of claim 2, wherein the C content is 0.15 to 0.4 wt%, the Mn content is 1.3 to 2.5 wt%, the Si content is 0.2 to 1.8 wt%, the Al content is 1.5 wt%, the Ti content is 0.03 wt%, and the Nb content is 0.03 wt%.

4. 2. The high strength hot rolled steel sheet product of claim 1, wherein the ferrite comprises at least 50% by volume and the retained austenite comprises 5 to 25% by volume.

5. A high strength hot rolled steel sheet product as claimed in claim 1, wherein the retained austenite has an average grain size of less than 10 microns.

6. A high strength hot rolled steel sheet product as claimed in claim 5, wherein the retained austenite has an average grain size of less than 1 micron.

7. A high strength hot rolled steel sheet product as claimed in claim 5, wherein the hot rolled steel sheet product contains less than 15 volume % fresh martensite.

8. The high strength hot rolled steel sheet product of claim 1, wherein the hot rolled steel sheet product has an ultimate tensile strength of 720 to 1,100 MPa and a total elongation of at least 20%.

9. 2. The high strength hot rolled steel sheet product of claim 1, wherein the UTS·TE is at least 27,000 MPa%.

10. The high strength hot rolled steel sheet product of claim 1, further comprising a zinc-based coating on said hot rolled steel sheet product.

11. 1. A method for producing a high strength hot rolled steel plate product comprising a steel containing 0.12 to 0.5 wt. % C, 1 to 3 wt. % Mn, 0.8 to 3 wt. % of a combination of Si and Al, with the balance being Fe and unavoidable impurities, comprising: subjecting the hot rolled steel sheet product to a first step annealing process to obtain a martensite-based microstructure; and subjecting the hot rolled steel plate product to a second step process, wherein the second step process comprises soaking the hot rolled steel plate product in an intercritical transformation zone at a temperature of 720-850°C and then holding the hot rolled steel plate product at a temperature of 360-445°C, wherein the hot rolled steel plate product comprises ferrite grains and substantially equiaxed retained austenite grains having an average aspect ratio of less than 3:1, and has an ultimate tensile strength and total elongation (UTS.TE) combination greater than 25,000 MPa% and a hole expansion ratio greater than 35%.

12. The method of claim 11, wherein the first step annealing process is carried out at a temperature above 820°C.

13. The method of claim 11, wherein the first step annealing process is carried out at a temperature of 830 to 940°C.

14. The method of claim 11, wherein the second step soaking process is carried out at a temperature of 720 to 850°C, and the second step holding process is carried out at a temperature of 370 to 440°C.

15. 12. The method of claim 11, wherein the hot rolled steel product is cooled to a temperature below 300°C between the first step process and the second step process.

16. 12. The method of claim 11, wherein the hot rolled steel product is maintained at a temperature greater than 300°C between the second step soaking process and the second step holding process.

17. The method of claim 11, wherein the first step annealing process is performed in a continuous annealing line and the second step process is performed in a continuous annealing line.

18. 18. The method of claim 17, wherein the same continuous annealing line is used for both the first-step annealing process and the second-step process.

19. 18. The method of claim 17, wherein separate continuous annealing lines are used for the first-step annealing process and the second-step process.

20. The method of claim 11, wherein the first step annealing process is carried out in a continuous annealing line and the second step process is carried out in a continuous galvanizing line.

21. The method of claim 11, further comprising electrolytically coating the hot rolled steel sheet product with a zinc-based coating.

22. 12. The method of claim 11, wherein the hot rolled steel sheet product comprises up to 2 wt.% Si and up to 2 wt.% Al, and further comprises up to 0.05 wt.% Ti and up to 0.05 wt.% Nb.

23. 23. The method of claim 22, wherein the C content is 0.15-0.4 wt%, the Mn content is 1.3-2.5 wt%, the Si content is 0.2-1.8 wt%, the Al content is up to 1.5 wt%, the Ti content is up to 0.03 wt%, and the Nb content is up to 0.03 wt%.

24. 12. The method of claim 11, wherein the ferrite comprises at least 50% by volume and the retained austenite comprises 5-25% by volume, and the retained austenite grains have an average aspect ratio of less than 2:

1.

25. The method of claim 11, wherein the retained austenite has an average grain size of less than 10 microns.

26. The method of claim 25, wherein the retained austenite has an average grain size of less than 1 micron.

27. The method of claim 11, wherein the hot rolled steel sheet product contains less than 15% by volume of fresh martensite.

28. The method of claim 11, wherein the hot rolled steel sheet product has an ultimate tensile strength of 720 to 1,100 MPa and a total elongation of at least 20%.

29. The method of claim 11, further comprising applying a zinc-based coating to the hot rolled steel sheet product.

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