High strength steel product and annealing method for producing the high strength steel product

Abstract

The present invention provides a steel sheet product with a controlled composition that undergoes a two-step annealing process to produce a sheet product with a desired microstructure and favorable mechanical properties such as high strength and ultra-high formability. The steel sheet product can be cold rolled or hot rolled. The steel processed according to the present invention exhibits favorable ultimate tensile strength and total elongation (UTS·TE) combination properties and can fall into the category of third generation advanced high strength steels, which are desirable in various industries including automotive manufacturers.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application is a continuation-in-part of U.S. Patent Application No. 15 / 591,344 filed on May 10, 2017, which claims priority to U.S. Provisional Application No. 62 / 334,189 filed on May 10, 2016 and U.S. Provisional Application No. 62 / 396,602 filed on September 19, 2016, and U.S. Patent Application No. 10,385,419. This application is also a continuation-in-part of U.S. Patent Application No. 16 / 459,757 filed on July 2, 2019, which is a continuation-in-part of U.S. Patent Application No. 15 / 591,344. All of the foregoing applications are incorporated herein by reference. Field of the Invention

[0003] The present invention relates to high strength steel products having advantageous properties, and to annealing methods for producing such products. Background Art

[0004] Over the past few years, the global steel industry has focused on the development of third generation advanced high strength steels (AHSS) for the automotive market. This third generation steel has a favorable balance of tensile strength and elongation, typically in the UTS·TE range of about 20000 MPa% or greater. However, the steel industry is in a difficult period of commercializing 3rd generation AHSS due to the difficulties most methods require when manufacturing such steels with conventional steelmaking equipment due to the high alloy content, such as typically greater than 4 wt. %, of manganese. In addition, currently available AHSS is difficult to weld by techniques such as spot welding, difficult to coat with zinc-based electroplated coatings, and difficult to manufacture into thin gauge sheets required for large-scale applications. Summary of the invention

[0005] The present invention provides a steel sheet product with a controlled composition, which undergoes a two-step annealing process to produce a sheet product with a desired microstructure and favorable mechanical properties such as high strength and ultra-high formability. The steel sheet product can be cold rolled or hot rolled. The steel processed according to the present invention exhibits a favorable ultimate tensile strength and total elongation (UTS·TE) combination of properties, such as greater than 25000 MPa-% when tested using a standard subsize ASTM or full-size JIS tensile test procedure. In addition, the steel prepared according to the present invention exhibits a favorable combination of TE and hole expansion, i.e., both global formability 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.

[0006] One aspect of the present invention is to provide a high strength rolled steel sheet product comprising 0.12-0.5 wt. % C, 1-3 wt. % Mn and 0.8-3 wt. % Si in combination with Al, wherein the steel sheet product has undergone a two-step annealing process, comprises ferrite and substantially equiaxed retained austenite grains having an average aspect ratio of less than 3:1, and has a combined ultimate tensile strength and total elongation (UTS·TE) greater than 25000 MPa%.

[0007] Another aspect of the present invention is to provide a method for preparing a high strength rolled steel sheet product comprising 0.12-0.5 wt. % C, 1-3 wt. % Mn and 0.8-3 wt. % Si in combination with Al. The method comprises subjecting the steel sheet product to a first step annealing process to achieve a predominantly martensitic microstructure and subjecting the steel sheet product to a second step process comprising soaking the intercritical sheet product at a temperature of 720-850°C and thereafter maintaining the sheet product at a temperature of 370-445°C.

[0008] These and other aspects of the invention will become more apparent from the following description.

[0009] BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 A temperature versus time plot is included illustrating a two-step annealing method according to an embodiment of the present invention.

[0011] Figure 2 A temperature versus time curve is included illustrating a two-step annealing process according to another embodiment of the present invention.

[0012] Figure 3 is a temperature versus time curve illustrating a two-step annealing process combining a two-step thermal process with an optional zinc-based hot dip coating operation in a single production facility.

[0013] Figure 4 is a plot of temperature versus time for the second step of an annealing process defining soak and hold zones in a thermal cycle according to an embodiment of the present invention.

[0014] Figure 5 and 6 is an electron backscatter diffraction (EBSD) micrograph illustrating the microstructure of a high strength steel sheet product according to an embodiment of the present invention.

[0015] Figure 7 yes Figure 1 The optical micrograph of a steel sheet product that has undergone a thermal process shown in , shows darker ferrite grains and lighter austenite grains.

[0016] Figure 8 Yes Description Figure 7 A histogram showing the aspect ratio of austenite grains.

[0017] Fig. 9 and 10 is a graph of a high strength steel sheet product showing austenite and ferrite grain size distribution according to an embodiment of the present invention.

[0018] Fig.11 is an EBSD micrograph, indicating Figure 1 The microstructure of the processed high-strength steel sheet product is shown in FIG.

[0019] Fig.12 and 13 is an EBSD micrograph showing Figure 2 The processed steel sheet products are shown in Figure 2.

[0020] Fig.14 Yes Figure 3 EBSD micrograph of the processed steel sheet product is shown in FIG.

[0021] Fig.15 is a graph of total elongation versus ultimate tensile strength for a high strength steel sheet product of the present invention compared to other steel sheet products processed outside the scope of the present invention.

[0022] Fig.16 is a graph of total elongation versus ultimate tensile strength for high strength steel products produced in plant trials according to embodiments of the present invention.

[0023] Fig.17 2 is a plot of temperature versus time for cold rolled and hot rolled substrates processed using a thermal cycle according to an embodiment of the present invention.

[0024] Fig.18 yes Fig.17 EBSD micrograph of a cold rolled steel substrate that has undergone a thermal process, shown in Figure 2, shows darker ferrite grains and brighter retained austenite grains.

[0025] Fig.19 yes Fig.17 EBSD micrograph of a hot rolled steel substrate that has undergone thermal processing, shown in Figure 2, shows darker ferrite grains and lighter retained austenite grains.

[0026] Details

[0027] The high strength steel sheet product of the present invention has a controlled composition, which, together with a controlled annealing process, produces a desired microstructure and favorable mechanical properties including high strength and ultra-high formability. In some embodiments, the steel composition may include carbon, manganese and silicon, as well as 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.

[0028] Table 1

[0029] Steel composition (weight %)

[0030] 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

[0031] In addition to the amounts of C, Mn, Si, Al, Ti, and Nb listed in Table 1, the steel composition may include trace or impurity amounts of other elements, such as maximum 0.015 S, maximum 0.03 P, maximum 0.2 Cu, maximum 0.02 Ni, maximum 0.2 Cr, maximum 0.2 Mo, maximum 0.1 Sn, maximum 0.015 N, maximum 0.1 V, and maximum 0.004 B. The term "substantially free" as used herein when referring to the composition of the steel sheet product means that the particular element or material is not intentionally added to the composition and is present only as an impurity or in trace amounts.

[0032] In the steel sheet product of the present invention, C provides increased strength and promotes the formation of retained austenite. Mn provides hardening and acts as a solid solution strengthener. Si inhibits the precipitation of iron carbides during heat treatment and improves austenite retention. Al inhibits the precipitation of iron carbides during heat treatment and improves austenite retention. Ti and Nb can act as strength enhancing grain refiners.

[0033] In some embodiments, Al may be present in an amount of at least 0.1 wt %, or at least 0.2 wt %. For example, in some embodiments, Al may be present in an amount of 0.5-1.2 wt %, or 0.7-1.1 wt %. Alternatively, the steel sheet product may be substantially free of Al.

[0034] The steel sheet product having the composition as described above was subjected to a two-step annealing process as described more fully below. The resulting steel product was found to have advantageous mechanical properties including desirable ultimate tensile strength, high elongation, high lambda value, high bendability and high yield ratio (YS / UTS).

[0035] In some embodiments, the steel sheet product has an ultimate tensile strength (UTS) in the range of 700-1100 MPa or greater. In some embodiments, the steel sheet product has an ultimate tensile strength greater than 700 MPa, such as 720-1100 MPa or 750-1050 MPa.

[0036] In some embodiments, the steel sheet product has a total elongation (TE) 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%.

[0037] The steel sheet product may have a lambda value as measured by a standard hole expansion test typically greater than 20%, such as greater than 25%, or greater than 30%, or greater than 35%. The hole expansion ratio or lambda may be greater than 20%, such as 22-80% or 25-60%.

[0038] In some embodiments, enhanced values ​​of both total elongation (TE) and hole expansion (λ) result in a steel sheet product that exhibits good global and local formability.

[0039] The present steel sheet products observe a strength-to-elongation balance (UTS·TE) greater than 25,000, placing them within the category of third generation steels that are highly desired by multiple industries such as the automotive industry. In some embodiments, the UTS·TE value may be greater than 27,000, or greater than 30,000, or greater than 35,000.

[0040] According to some embodiments of the present invention, the final microstructure of the steel sheet product may mainly contain ferrite, such as at least 50% up to 80% or more, with a smaller amount of retained austenite, such as 5-25%, and a trace amount of newly formed martensite, such as 0-10% or 15%. The amount of ferrite, austenite and martensite can be determined by standard EBSD techniques. Alternatively, the retained austenite content can be determined by a magnetic saturation method. Unless otherwise specified herein, the volume % of retained austenite is determined by EBSD techniques.

[0041] In some embodiments, the retained austenite accounts for 1-25% by volume, such as 5-20% by volume. The amount of fresh martensite may account for less than 15% by volume, or less than 10% by volume, or less than 5% by volume. In some embodiments, the steel sheet product is substantially free of fresh martensite. It is found that when the amount of fresh martensite is greater than 15%, the hole expansion value is significantly reduced, such as the local formability is significantly reduced.

[0042] As described below, at least a portion of ferrite can be formed by tempering and / or recrystallization of martensite during the heating stage, or by decomposition of austenite during the cooling and holding stage of the second annealing process. Some ferrites can be considered to be bainitic ferrites. Ferrite, austenite and martensite phases are fine-grained, for example, with an average grain size of less than 10 microns, for example, less than 5 microns or less than 3 microns. For example, the ferrite grain size can range from less than 10 microns, for example, less than 8 microns or less than 6 microns. The average austenite grain size can range from less than 2 microns, for example, less than 1 micron or less than 0.5 micron. When present, the martensite grain size can range from less than 10 microns, for example, less than 8 microns or less than 6 microns.

[0043] The austenite grains may be substantially equiaxed, for example having an average aspect ratio of less than 3:1 or less than 2:1, for example about 1:1. It has been found that an amount of retained austenite less than about 5% results in a significantly reduced total elongation (TE). It has also been found that an amount of retained austenite greater than 25% can only be obtained at very high carbon levels, which results in poor weldability.

[0044] In some embodiments of the present invention, a two-step annealing method is used to produce an advanced high strength steel product having advantageous mechanical properties such as those described above. Within each of the first and second annealing steps, a variety of methods for performing heat treatment may be used. Figure 1-3 An example of a two-step annealing method is shown in and described below. Figure 1 A continuous annealing line (CAL) is presented, followed by a continuous annealing line (CAL) preparation route. Figure 2 Presenting the CAL plus continuous plating line (CGL) preparation route. Figure 3 Presents a specially designed production line that allows the CAL+CAL or CAL+CGL steps to occur in a single device. Figure 3 A direct fired furnace (DFF) followed by a radiant tube (RT) furnace embodiment is shown in FIG, but other embodiments such as all radiant tubes, electric radiant heating, etc. may be used to achieve the desired thermal cycle.

[0045] Step 1

[0046] The purpose of the first step of the annealing method is to achieve a martensitic microstructure in the cold rolled or hot rolled steel sheet product. In the first annealing stage of the first step, an annealing temperature greater than the A3 temperature can typically be used, for example, an annealing temperature of at least 820°C can be used. In some embodiments, the first stage annealing temperature can typically range from 830-980°C, for example, 830-940°C, or 840-930°C, or 860-925°C. In some embodiments, the peak annealing temperature can typically be maintained for at least 20 seconds, for example, 20-500 seconds, or 30-200 seconds. Heating can be achieved by conventional techniques such as non-oxidizing or oxidizing direct flame furnaces (DFF), oxygen-enriched DFI, induction, gas radiant tube heating, electric radiation heating, etc. Examples of heating systems that may be suitable for use in the methods of the present invention are disclosed in U.S. Patent 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, all to Fives Stein. Additional examples of heating systems that may be suitable for use in the methods of the present invention include U.S. Patent No. 7,384,489, to Drever International, and U.S. Patent No. 9,096,918, to Nippon Steel and Sumitomo Metal Corporation. Any other suitable known type of heating system and method may be suitable for use in steps 1 and 2.

[0047] In the first stage, after reaching the peak annealing temperature and holding for a desired period of time, the cold-rolled or hot-rolled steel sheet is quenched to room temperature, or to a controlled temperature greater than room temperature, as described more fully below. The quenching temperature need not be room temperature, but should be below the martensite start temperature (M s ) and preferably below the martensite finish temperature (M F ), thereby forming a microstructure that is primarily martensite. In some embodiments, between the first step process and the second step process, the steel sheet product may be cooled to a temperature below 300° C., such as below 200° C.

[0048] Quenching can be achieved by conventional techniques, such as water quenching, immersed knife / nozzle water quenching, gas cooling, rapid cooling using a combination of cold, warm or hot water and gas, aqueous solution cooling, other liquid or gas fluid cooling, rapid chill roll quenching, water mist spray, wet flash cooling, non-oxidizing wet flash cooling, etc. Quenching rates of 30-2000°C / second can typically be used.

[0049] Various types of cooling and quenching systems and methods known to those skilled in the art may be suitable for use in the methods of the present invention. Suitable cooling / quenching systems and methods routinely used on a commercial basis may include water quenching, water mist cooling, dry instantaneous and wet instantaneous, oxidative and non-oxidative cooling, alkane fluid to gas phase change cooling, hot water quenching (including two-step water quenching), roller quenching, high percentage hydrogen or helium jet cooling, etc. For example, dry instantaneous and / or wet instantaneous oxidative and non-oxidative cooling / quenching such as disclosed in published PCT application number WO2015 / 083047 of Fives Stein may be used. Other Fives Stein patent documents describing cooling / quenching systems and methods that may be suitable for use in the methods 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. US2009 / 0158975 A1, US2009 / 0315228 A1, and US2011 / 0266725 A1. Other examples of cooling / quenching systems and methods that may be suitable for use in the methods of the present invention include those disclosed in 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.

[0050] In some embodiments, after the first stage peak annealing temperature is reached and the steel is quenched to form martensite, the martensite may be optionally tempered to soften the steel slightly to make further processing more feasible. Tempering occurs by raising the temperature of the steel in the range of room temperature to about 500° C. and holding for up to 600 seconds. If tempering is used, the tempering temperature may be held constant, or may vary within this preferred range.

[0051] After tempering, the temperature is ramped down to room temperature. The rate of such ramping down may typically range from 1-40°C / sec, for example 2-20°C / sec. In the case of a single-pass equipment furnace, such as Figure 3 In this case, tempering may not be necessary.

[0052] Step 2

[0053] The second step of the annealing method may include a first stage performed at a relatively high annealing temperature and a second stage performed at a relatively low temperature. Figure 4 These stages are defined as the "soak" and "hold" regions of the secondary anneal as described in . The temperature is controlled so as to promote the formation of the desired microstructure in the final product.

[0054] In the first annealing stage of the second step, a soaking zone temperature between A1 and A3 may be used, for example, an annealing temperature of at least 720° C. may be used. In some embodiments, the soaking zone temperature may typically range from 720-850° C., such as 760-825° C. In some embodiments, the peak annealing temperature may typically be maintained for at least 15 seconds, such as 20-300 seconds, or 30-150 seconds.

[0055] During the first phase of the second step, the s The soaking zone temperature is achieved by heating the steel at a relatively low temperature, such as room temperature, at an average rate of 0.5-50°C / second, such as about 2-20°C / second. In some embodiments, the ramp up may be performed for 25-800 seconds, such as 100-500 seconds. The first stage heating of the second step may be achieved by any suitable heating system or method, such as using radiation heating, induction heating, direct flame furnace heating, etc.

[0056] After reaching the soaking zone temperature and maintaining the desired time period, the steel can be cooled to a controlled temperature greater than room temperature to the holding zone. In some embodiments, the steel sheet product is maintained at a temperature greater than 300°C between the second soaking process and the second holding process. Cooling from the soaking to the holding zone can be achieved by conventional techniques such as water cooling, gas cooling, etc. An average cooling rate of 5-400°C / second can typically be used. Any suitable type of cooling and quenching system may be suitable for use in cooling from the soaking temperature to the holding temperature, including those described above.

[0057] According to an embodiment of the present invention, the holding zone step is carried out at a typical temperature of 360-445° C., such as 370-440° C. The holding zone may be maintained for up to 800 seconds, such as 30-600 seconds.

[0058] The holding zone temperature may be kept constant or may vary slightly within a preferred temperature range. After holding, if the steel is to be hot dip coated, the steel may be reheated, for example by induction or other heating methods, to enter the hot dip coating tank at an appropriate temperature for obtaining good coating results.

[0059] In some embodiments, after the temperature of the insulation zone is maintained for a desired period of time, the temperature can be ramped down to room temperature. Such a ramp down can be optionally performed for 10-1000 seconds, such as about 20-500 seconds. The rate of such a ramp down can typically range from 1-1000°C / second, such as 2-20°C / second.

[0060] According to some embodiments, one or both of the first and second annealing processes may be performed on a continuous annealing line (CAL).After undergoing the CAL+CAL process, the steel may be electroplated to produce a zinc-based coated product.

[0061] In some embodiments, the annealed steel sheet is hot dip plated at the end of the holding zone. The plating temperature may typically range from 440-480°C, such as 450-470°C. In some embodiments, the annealed steel sheet may be hot dip plated at the end of the holding zone. Figure 2 The electroplating step is performed on a continuous electroplating line (CGL) as shown in the CAL+CGL process as part of the second step annealing process. This CAL+CGL process can be used to prepare zinc-based or zinc alloy-based hot-dip electroplated products or to reheat after coating to prepare electroplated iron-zinc coating diffusion annealing type coated products. An optional nickel-based coating step can occur between the CAL and CGL steps in the process to improve the zinc coating properties. The use of a continuous electroplating line in the second step improves the efficiency of preparing the coated GEN3 product relative to the use of a CAL+CAL+EG route.

[0062] Electroplated products or hot-dip coated products based on zinc alloys can also be made on specially designed CGLs where two-step annealing can take place in a single line, such as Figure 3 In this case, diffusion annealing of the zinc coating may also be an option. In addition, a single production plant may be specially designed and built to combine the two-step thermal process to produce the uncoated third generation steel as defined in the present invention.

[0063] The following examples are intended to illustrate various aspects of the present invention and are not intended to limit the scope of the invention.

[0064] Example 1

[0065] The cold-rolled steel sheet No. 1 sample having the composition listed in Table 2 was subjected to the following Figure 1 The two-step annealing method described in Figure 5 and 6 The microstructure of the obtained product is shown in FIG. EBSD technique using commercially available EDAX orientation imaging microscopy software is shown in Figure 5 Medium dark ferrite grains and bright austenite grains.

[0066] Example 2

[0067] The cold-rolled steel sheet No. 2 sample having the composition listed in Table 2 was subjected to the following Figure 1 The two-step annealing method described in Fig.11 The microstructure of the obtained product is shown in Table 2. The mechanical properties of sample No. 2 are listed in Table 2. Fig. 9 and 10 The grain size distribution of austenite and ferrite is shown in Figure 2. The average austenite grain size is less than 1 micron and the average ferrite grain size is less than 10 microns.

[0068] The microstructure includes about 80 volume % ferrite having an average grain size of about 5 microns, about 10 volume % retained austenite having substantially equiaxed grains and an average grain size of about 0.5 microns, and about 10 volume % fresh martensite having an average grain size of about 5 microns. The mechanical properties of Sample No. 1 are listed in Table 2 below.

[0069] Example 3

[0070] The cold-rolled steel sheet No. 3 sample having the composition listed in Table 2 was subjected to the following Figure 2 The two-step annealing method described in Fig.12 and 13 The microstructure of the obtained product is shown in Fig.13 In the example, austenite is bright in color and ferrite is dark in color. The mechanical properties of sample No. 3 are listed in Table 2.

[0071] Example 4

[0072] Sample No. 4 of the cold-rolled steel sheet having the composition listed in Table 2 was subjected to the following Figure 3 The two-step annealing method described in Fig.14 The microstructure of the obtained product is shown in Fig.14 In the example, austenite is bright in color and ferrite is dark in color. The mechanical properties of sample No. 4 are listed in Table 2.

[0073] Example 5

[0074] The cold-rolled steel sheet No. 5 sample having the composition listed in Table 2 was subjected to the following Figure 1 The mechanical properties of sample No. 5 are listed in Table 2.

[0075] Example 6

[0076] The cold-rolled steel sheet No. 6 sample having the composition listed in Table 2 was subjected to the following Figure 1 The mechanical properties of sample No. 6 are listed in Table 2. Figure 7 Yes Display Figure 2 The optical image of the microstructure of steel sample No. 6 is shown in FIG. Figure 1 The two-step annealing method shown in Figure 7 In the micrograph, the dark areas are ferrite grains, while the bright areas are austenite grains. Figure 8 Yes Description Figure 7 A plot of the aspect ratio of austenite grains shown in . Figure 7 The optical images were used to determine the aspect ratio of the austenite grains, where commercially available software was used for image analysis. Figure 7The average aspect ratio of the austenite grains is shown to be less than 3:1.

[0077] Example 7

[0078] The cold-rolled steel sheet No. 7 sample having the composition listed in Table 2 was subjected to the following Figure 2 The mechanical properties of sample No. 7 are listed in Table 2.

[0079] Example 8

[0080] The cold-rolled steel sheet No. 8 sample having the composition listed in Table 2 was subjected to the following Figure 3 The mechanical properties of sample No. 8 are listed in Table 2.

[0081] The steels in Examples 1-8 exhibited UTS levels in the range of 700 to 1100 MPa.

[0082] Comparative Examples 1-4

[0083] The cold-rolled steel sheets C1-C4 samples having the compositions listed in Table 2 were subjected to the following Figure 1 The two-step annealing method described in . The mechanical properties of samples No. C1-C4 are listed in Table 2. The steels in Comparative Examples 1-4 exhibited UTS levels of less than 700 MPa.

[0084] Comparative Examples 5-8

[0085] The cold rolled steel sheets C5-C8 samples having the compositions listed in Table 2 were subjected to the following Figure 1 The two-step annealing method described in . The mechanical properties of samples C5-C8 are listed in Table 2. The steels in Comparative Examples 5-8 exhibit UTS levels greater than 1100 MPa.

[0086] Comparative Examples 9-11

[0087] The cold rolled steel sheets C9-C11 samples having the compositions listed in Table 2 were subjected to Figure 1 A similar two-step annealing method as described in , except that the soaking or holding temperature in the second annealing is within the preferred range of the invention. The mechanical properties of samples C9-C11 are listed in Table 2.

[0088] Comparative Example 12

[0089] The cold rolled sheet C12 sample having the composition listed in Table 2 was subjected to Figure 2 A similar two-step annealing method as described in , except that the temperature of the holding zone in the second annealing is within the preferred range of the invention. The mechanical properties of sample C12 are listed in Table 2.

[0090]

[0091]

[0092] Fig.15 The total elongation (TE) and ultimate tensile strength (UTS) of samples 1-8 of Examples 1-8 and samples C1-C12 of Comparative Examples C1-C12 are plotted. Fig.15 A line corresponding to a UTS·TE of 25000 is roughly drawn in FIG. As can be seen, the high strength steel sheet samples prepared according to the present invention have an excellent combination of strength and elongation relative to the comparative samples, i.e., high total elongation properties at high UTS levels are observed for the inventive examples. Samples 1 to 8 steels fall into the category of third generation advanced high strength steels, which are highly desired by the automotive and other industries.

[0093] Example 9

[0094] The factory tests were performed using the CAL+CAL or CAL+CGL method for the samples labeled M1-M5 in Table 3 below. For samples M1, M2 and M5, Figure 1 The CAL+CAL processing time and temperature shown in . For samples M3 and M4, use Figure 2 CAL+CGL processing time and temperature shown in.

[0095] Table 3

[0096] Factory test results

[0097]

[0098] Fig.16 The strength-elongation balance of the factory test materials was shown, all meeting the minimum UTS·TE of 25,000. The test materials exhibited lambda values ​​greater than 20%.

[0099] Example 10

[0100] Cold-rolled and hot-rolled steel sheets corresponding to Sample Nos. 9A to 12B in Table 4 having a composition of 0.23 wt. % C, 2.3 wt. % Mn, 0.6 wt. % Si, and 0.8 wt. % Al were subjected to the following conditions: Fig.17 The two-stage annealing method shown in . In Table 4, the cold rolled samples are listed as "CR" substrate type, and the hot rolled samples are listed as "HR" substrate type. The mechanical properties of Samples No. 9A-12B are listed in Table 4. The hot rolled substrate samples show excellent YS, UTS, TE and hole expansion properties comparable to the cold rolled samples, showing that the hot rolled substrate directly processed into the two-stage annealing method can produce third generation AHSS properties. In addition, as Fig.18 and 19 The EBSD phase distribution shown in (where the retained austenite grains are brighter than the ferrite grains) indicates that similar austenite content, distribution and morphology are observed for the hot rolled material when compared to the cold rolled material. Fig.18 The austenite content of cold rolled sample 11A is shown and Fig.19 The austenite content is shown for hot rolled sample 12 A. A fine, mainly equiaxed distribution of austenite is observed in both microstructures.

[0101] Table 4

[0102]

[0103] As used herein, "comprising," "containing," and similar terms are to be understood as synonymous with "comprising," and are therefore open ended and do not exclude the presence of additional undescribed or undocumented elements, materials, phases, or process steps. As used herein, "consisting of" is to be understood in the context of the present application to exclude the presence of any unspecified elements, materials, phases, or process steps. As used herein, "consisting essentially of" is to be understood in the context of the present application to include the specified elements, materials, phases, or process steps, when applicable, and also include any unspecified elements, materials, phases, or process steps that do not materially affect the basis or novel characteristics of the invention.

[0104] 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. Any numerical values, however, inherently contain certain errors necessarily resulting from the standard variations found in their respective testing measurements.

[0105] In addition, it should be understood that any numerical range recited herein is intended to include all sub-ranges included therein. For example, a range of "1 to 10" is intended to include all sub-ranges between the recited minimum value of 1 and the recited maximum value of 10 (and including the recited minimum value of 1 and the recited maximum value of 10), i.e., all sub-ranges having a minimum value equal to or greater than 1 and a maximum value equal to or less than 10.

[0106] In this application, the use of the singular includes the plural and the plural includes the singular unless specifically indicated otherwise. Additionally, in this application, the use of "or" means "and / or" unless specifically indicated otherwise, even though "and / or" may be explicitly used in some instances. In this application and the appended claims, the articles "a," "an," and "the" include plural referents unless expressly and unequivocally limited to one referent.

[0107] 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 numerical variations in the details of the invention may be made without departing from the invention.

Claims

1. A high strength rolled steel sheet product comprising 0.12-0.5 wt. % C, 1-3 wt. % Mn and 0.8-3 wt. % Si in combination with Al, wherein the steel sheet product has been subjected to a hot rolling and two-step annealing process, the annealing process comprising: subjecting the steel sheet product to a first annealing process to achieve a microstructure that is primarily martensite; and subjecting the steel sheet product to a second process comprising soaking the sheet product in an intercritical state at a temperature of 720 to 850° C., thereafter maintaining the sheet product at a temperature of 370 to 445° C., wherein the steel sheet product comprises ferrite and substantially equiaxed retained austenite grains having an average aspect ratio of less than 2:1 and an average grain size of less than 1 micron, and has a combined ultimate tensile strength and total elongation (UTS∙TE) greater than 25000 MPa%; wherein ferrite accounts for at least 50% by volume, and retained austenite accounts for 5 to 25% by volume, and has a hole expansion ratio greater than 35%.

2. The high strength rolled steel sheet product according to claim 1, wherein Si accounts for up to 2 wt%, Al accounts for up to 2 wt%, and the rolled steel sheet product further comprises up to 0.05 wt% Ti and up to 0.05 wt% Nb.

3. The high strength rolled steel sheet product according to claim 2, wherein C accounts for 0.15-0.4 wt%, Mn accounts for 1.3-2.5 wt%, Si accounts for 0.2-1.8 wt%, Al accounts for up to 1.5 wt%, Ti accounts for up to 0.03 wt%, and Nb accounts for up to 0.03 wt%.

4. The high strength rolled steel sheet product according to claim 1, wherein the rolled steel sheet product comprises less than 15 volume % of freshly formed martensite.

5. The high strength rolled steel sheet product according to claim 1, wherein the rolled steel sheet product has an ultimate tensile strength of 720-1100 MPa and has a total elongation of at least 20%.

6. The high strength rolled steel sheet product according to claim 1, wherein UTS∙TE is at least 27000 MPa%.

7. The high strength rolled steel sheet product according to claim 1, further comprising a zinc-based coating on the rolled steel sheet product.

8. A method of producing a high strength rolled steel sheet product comprising 0.12-0.5 wt. % C, 1-3 wt. % manganese and 0.8-3 wt. % Si and Al in combination, the method comprising: subjecting the hot rolled steel sheet product to a first step annealing process to achieve a predominantly martensitic microstructure; and The steel sheet product is subjected to a second process comprising soaking the sheet product in an intercritical state at a temperature of 720-850°C, and thereafter maintaining the sheet product at a temperature of 360-445°C, wherein the steel sheet product comprises ferrite and substantially equiaxed retained austenite grains having an average aspect ratio of less than 2:1 and an average grain size of less than 1 micron, and has a combined ultimate tensile strength and total elongation (UTS∙TE) greater than 25000 MPa%; wherein ferrite accounts for at least 50% by volume, and retained austenite accounts for 5 to 25% by volume, and has a hole expansion ratio greater than 35%.

9. The method according to claim 8, wherein the first annealing process is performed at a temperature greater than 820°C.

10. The method according to claim 8, wherein the first annealing process is performed at a temperature of 830-940°C.

11. The method according to claim 8, wherein the second soaking process is performed at a temperature of 720-850°C, and the second heat preservation process is performed at a temperature of 370-440°C.

12. The method according to claim 8, wherein the steel sheet product is cooled to a temperature below 300°C between the first annealing process and the second process.

13. The method of claim 8, wherein the steel sheet product is maintained at a temperature greater than 300°C between the second soaking process and the second holding process.

14. The method of claim 8, wherein the first step annealing process is performed on a continuous annealing line, and the second step process is performed on a continuous annealing line.

15. The method of claim 14, wherein the same continuous annealing line is used for both the first annealing process and the second process.

16. The method of claim 14, wherein separate continuous annealing lines are used for the first annealing process and the second annealing process.

17. The method of claim 8, wherein the first annealing process is performed on a continuous annealing line, and the second process is performed on a continuous electroplating line.

18. The method of claim 8, further comprising electrolytically coating the rolled steel sheet product with a zinc-based coating.

19. The method of claim 8, wherein Si comprises up to 2 wt%, Al comprises up to 2 wt%, and the rolled steel sheet product further comprises up to 0.05 wt% Ti and up to 0.05 wt% Nb.

20. The method of claim 19, wherein C accounts for 0.15-0.4 wt%, Mn accounts for 1.3-2.5 wt%, Si accounts for 0.2-1.8 wt%, Al accounts for up to 1.5 wt%, Ti accounts for up to 0.03 wt%, and Nb accounts for up to 0.03 wt%.

21. The method of claim 8, wherein the rolled steel sheet product comprises less than 15 volume % freshly produced martensite.

22. The method of claim 8, wherein the rolled steel sheet product has an ultimate tensile strength of 720-1100 MPa and has a total elongation of at least 20%.

23. The method of claim 8, further comprising applying a zinc based coating on the rolled steel sheet product.

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