High-strength galvanized steel sheet and galvannealed electroplated steel sheet, and their manufacturing method

Abstract

This ultra-high strength steel offers an improved process window for galvanizing and galvannealed electroplating thanks to a new alloying concept. In addition to the alloying concept, the steel contains a strengthened ferrite and overaged martensite structure with a controlled volume fraction of magnetic phase. The steel can be processed into sheet form and then galvanized or galvannealed electroplated.

Description

[Background technology]

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 451,994, filed March 14, 2023, entitled "High Strength Galvanized Steel Sheet and Alloyed Electrogalvanized Steel Sheet and Method for Manufacturing Same," the disclosure of which is incorporated herein by reference.

[0002] Automotive applications demand high-strength, high-ductility steels. The adoption of high-strength steels has led to the weight reduction of automobiles and improved crashworthiness. At least in the automotive steel market, new steels aim to meet the requirements of high ultimate tensile strength, excellent ductility, improved strain hardening behavior, formability, and the ability to be processed in galvanized (GI) and galvannealed (GA) coatings.

[0003] Steels with various strength levels have been proposed and used in the automotive market and elsewhere. One example is multiphase (DP) steel, which contains ferrite and island martensite phases. DP steels have relatively low yield strength (YS) and ultimate tensile strength (UTS), but offer excellent ductility and formability. Another example is multiphase (MP) steel, which contains ferrite, bainite, and martensite phases. MP steels have higher yield strength and ultimate tensile strength than DP steels, typically in the 700-1000 MPa range. Another example is complex-phase (CP) steel, which contains ferrite-bainite, martensite, retained austenite, and / or pearlite phases. CP steels have higher strength than DP and MP steels, typically in the 980-1299 MPa range. There are also single-phase (SP) steels, which contain microstructures with bainite or martensite and offer very high yield and ultimate tensile strengths, i.e., in the range of 1300-1700 MPa, but have relatively low ductility and formability compared to DP, MP, and CP steels.

[0004] As an example, U.S. Patent Publication US2013 / 008570 relates to a high-strength steel having an ultimate tensile strength of 1100 MPa, formability, a balance of strength and elongation, and bendability. The microstructure described in US2013 / 008570 is composed of 50% or more martensite, 15% or more ferrite-bainite, and 0-5% polygonal ferrite. The publication does not disclose the steel's ability to undergo galvanizing or electrogalvannealing processes.

[0005] International Patent Publication WO2012 / 153016 relates to a cold-rolled steel sheet having an ultimate tensile strength of more than 1000 MPa and an elongation of more than 12%. The microstructure described in WO2012 / 153016 consists of 5-15% martensite, 10-15% retained austenite, and 5-20% polygonal ferrite. The publication does not disclose the galvanizing-electrogalvannealing capability of this steel.

[0006] U.S. Patent No. 11,0047,020 relates to cold-rolled, hot-dipped steel with a maximum tensile strength of 980-1180 MPa. The microstructure of the steel described is 50-90% martensite and 5-50% ferrite + bainite. U.S. Patent No. 11,047,020 discloses a galvanized-galvannealed electrogalvanized steel with a maximum tensile strength of 1180 MPa.

[0007] U.S. Patent No. 8,840,834 relates to an ultra-high strength steel with a maximum tensile strength of 1400 MPa or more. The microstructure contains 80% or more tempered martensite with precipitated iron-based carbides, 5% or less ferrite, 10% or less bainite, and 5% or less retained austenite. However, as the galvannealing (GA) temperature of the steel of the '834 patent increases, the proportion of magnetic phases such as ferrite decreases, resulting in a decrease in GA capability. Furthermore, as the magnetic phase increases, the proportion of autotempered martensite increases, resulting in a decrease in tensile strength of the steel of the '834 patent.

[0008] Japanese Patent No. 2528387 relates to an ultra-high strength cold-rolled steel sheet that has a maximum tensile strength of at least 1500 MPa and good formability by annealing under specific conditions, quenching with water spray, and overaging. The steel in this Japanese patent is not suitable for galvannealed electroplating because the galvannealed electroplating temperature is much higher than the overaging temperature.

[0009] International Patent Publication WO 2021 / 176249 relates to ultra-high-strength cold-rolled, alloyed, electrogalvanized steel sheets with a tensile strength of 1450 MPa or greater. The steel structure is 80-90% martensite and the remainder ferrite and bainite, with 5% or greater ferrite and / or 5% or greater bainite. Intercritical annealing (IA) is employed between the Ac1 and Ac3 temperatures to obtain 5-15% ferrite for GA capacity. However, the steel described in WO 2021 / 176249 has a very narrow IA processing window, making it difficult to control the ferrite fraction. At low IA temperatures, GA capacity is maintained but the ultimate tensile strength is below 1450 MPa. At high IA temperatures, the ultimate tensile strength is higher than 1450 MPa, but GA capacity is lost. This publication does not consider the impact of the IA processing window on the balance between tensile strength and GA capacity.

[0010] While there are a variety of current steels, some of which are described above, there continues to be a need for improved alloyed electrogalvanized-galvanized steels with ultra-high strength (above 1480 MPa) and good ductility with a wider GI / GA process window to address the balance between tensile strength and GA capability. Meeting this need will improve manufacturing reliability, especially in facilities that employ induction heating. While various steels and methods of manufacturing steels have been made and used, it is believed that no one prior to the present inventors has made or used the invention as described herein. [Brief explanation of the drawings]

[0011] While the specification concludes with claims which particularly point out and distinctly claim the invention, it is believed the invention will be better understood from the following description of specific embodiments taken in conjunction with the accompanying drawings in which like reference numerals identify the same elements and in which: [Figure 1] Figure 1 is a schematic graph for achieving 20-65% martensite and tempered martensite at elevated temperatures by galvanizing and galvannealed electrogalvanizing using a fully austenitized and quenched-and-tempered (FA / QT-G / GA) approach. [Figure 2] Figure 2 is a schematic graph of the critical annealing (IA-GI / GA) approach, which involves a critical annealing process window between Ac3 and Ac1 to obtain 10-25% ferrite, followed by galvanizing and electrogalvannealing. [Figure 3] Figure 3 is a schematic graph of the critical annealing-quenching-tempering (IA / QT-GI / GA) approach, which has a critical annealing process window followed by a quenching-tempering process window with galvanizing and electrogalvannealing. [Figure 4] Figure 4 is a schematic diagram of the critical inter-annealing processing window showing that the unoptimized composition has a very narrow critical inter-annealing processing window, while the optimized composition has a critical inter-annealing processing window that is two to three times wider. The drawings are not intended to be limiting in any sense, and it is contemplated that the various embodiments of the invention may be practiced in a variety of other ways, including ways not necessarily depicted in the drawings. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several aspects of the invention and, together with the specification, serve to explain the principles of the invention, although it is understood that the invention is not limited to the precise arrangements shown. DETAILED DESCRIPTION OF THE INVENTION

[0012] The following description of specific examples of the present invention should not be used to limit the scope of the present invention. Other examples, features, aspects, embodiments, and advantages of the present invention will become apparent to those skilled in the art from the following illustrative description of one of the best modes contemplated for carrying out the present invention. As will be understood, the present invention is capable of other different and obvious embodiments without departing from the present invention. Accordingly, the drawings and description should be regarded as illustrative in nature and not restrictive.

[0013] As mentioned above, in conventional steelmaking, a trade-off exists between high strength and galvannealed electrogalvanization (GA) capability, with strength being sacrificed to achieve high GA capability. Certain embodiments of the present disclosure aim to resolve this trade-off by using a new alloying concept for ultra-high strength steel plates with minimum tensile strengths of 1480-1750 MPa and a new manufacturing method for such ultra-high strength galvanized (GI) and / or galvannealed electrogalvanized (GA) steels. This new alloying concept and manufacturing method broaden the GI / GA processing window and achieve a good combination of high strength and GA capability.

[0014] Certain embodiments of the present disclosure provide a combination of yield strength, tensile strength, ductility, and formability, providing an expanded GI / GA processing window and good manufacturing reliability not achievable with conventional steelmaking. In some examples, the steel resulting from the compositions and methods disclosed herein has a tensile strength of 1480-1750 MPa, a yield strength of 1050-1280 MPa, and a total elongation of greater than or equal to 7%. In this regard, embodiments of the present disclosure relate to ultra-high strength galvanized steel sheets and / or alloyed electrogalvanized steel sheets having high tensile strength and yield strength, as well as good ductility, formability, and alloyed electrogalvanization capabilities. In one embodiment, the alloy composition by weight is as follows, including the balance Fe and incidental impurities: 0.15-0.26% C, 2.10-3.60% Mn, 0.05-0.85% Si, 0.001-0.85% Al, 0.01-0.90% Cr, 0.01-0.50% Mo, 0.01-0.10% Ti, 0.01-0.04% Nb, 0.01-0.30% V, 0.0001-0.005% B, less than 0.01% N, less than 0.01% S, less than 0.05% P.

[0015] Three exemplary manufacturing approaches can be used with the alloying concept: (1) fully austenitized and quenched and tempered (FA / QT-Gi / GA), (2) intercritical annealed (IA-GI / GA), and (3) intercritical annealed, quenched and tempered (IA / QT-GI / GA). In IA-GI / GA and IA / QT-GI / GA, the steel structure consists of 10-25% ferrite, 75-90% martensite, and 3-10% retained austenite by area. In FA-GI / GA, the steel microstructure consists of 20-65% tempered martensite, 35-80% fresh martensite, and 3-10% retained austenite by area, combining high strength with excellent alloyed electrogalvanization capability.

[0016] To achieve good GA capability in ultra-high strength steels, a sufficient amount of magnetic phase, such as ferrite or martensite, must be present in the high-temperature structure to ensure the GA process. A higher proportion of magnetic phase improves GA capability. However, ferrite is a soft phase, and tempered or overaged martensite is not a hard phase. Achieving the ultra-high strength requires a higher proportion of hard phase. This can be achieved by increasing the strength of the ferrite and overaged martensite phases. In at least some embodiments described herein, available strengthening mechanisms include at least one or more of the following: (A) solid solution strengthening, such as with Mn, Si, Al, and Mo; (B) precipitation strengthening, such as with vanadium carbide, titanium carbide, and / or niobium carbide; (C) grain refinement strengthening; and / or (D) strain strengthening through temper rolling and / or elongation.

[0017] In addition to strengthening all phases, controlling the volume fraction of the soft (magnetic) phase also widens the QT or IA processing window to optimize GA capability and ultrahigh strength. In the FA / QT-GI / GA approach, the microstructure is engineered with 20-65%, preferably 35-50%, QT-tempered martensite for strong GA capability, 35-80%, preferably 65-50%, fresh martensite for ultrahigh strength, and 3-10%, preferably 5-8%, retained austenite for strain strengthening and good ductility. In the IA-GI / GA and IA / QT-GI / GA approaches, the microstructure is engineered with 10-25%, preferably 13-19%, ferrite for strong GA capability, 75-90%, preferably 80-75%, martensite for ultrahigh strength, and 3-10%, preferably 5-8%, retained austenite for strain strengthening and good ductility.

[0018] The following paragraphs provide at least some exemplary reasons for selecting the compositions and ranges of the alloy concepts described above. In some cases, the term "about" may be used when describing a range or value of a composition element. In this case, the term "about" should be given its broadest meaning as understood by one of ordinary skill in the art, or ±10% of the specified value if the broadest meaning as understood by one of ordinary skill in the art is unknown or inconclusive.

[0019] Carbon: 0.15% to 0.26%. Carbon is an essential element used to strengthen martensite. If the carbon content is lower than 0.15%, the minimum required tensile strength of 1480 MPa cannot be achieved. If the carbon content exceeds 0.26%, ductility decreases and weldability deteriorates. Furthermore, as the carbon content increases, the martensite start (Ms) temperature becomes very low, narrowing the QT window. Preferably, the carbon content is in the range of 0.18 to 0.22%.

[0020] Manganese: 2.10% or more and 3.60% or less. Manganese is an element that strengthens martensite and also solid-solution strengthens ferrite. To obtain a tensile strength of 1480 to 1750 MPa, it is necessary to ensure the desired amount of hard phase. If the manganese content is lower than 2.10%, the tensile strength will be lower than 1480 MPa. If the manganese content is higher than 3.60%, it may cause segregation of Mn, which may have an adverse effect on the tensile strength. Preferably, the manganese content is in the range of 2.60 to 3.30%.

[0021] Silicon: 0.05% to 0.85%. Silicon increases strength through solid solution strengthening and is also a ferrite stabilizer that increases the Ac3 temperature. Excessive addition reduces hot workability. Furthermore, silicon oxide is formed on the surface, which can impair melt paintability. In the IA-GI / GA or IA / QT-GI / GA process, the amount of silicon added must be optimized with the amount of aluminum added to widen the IA processing window. Preferably, the silicon content is in the range of 0.15 to 0.60%.

[0022] Aluminum: 0.001% to 0.80%. Aluminum forms AlN to prevent the formation of boron nitride. Aluminum is also a strong ferrite stabilizer, significantly increasing the Ac3 temperature. Excess aluminum leads to very high annealing temperatures, which creates manufacturing problems. Therefore, the aluminum content is jointly optimized with the silicon content to broaden the IA processing window. Preferably, the aluminum content is in the range of 0.15 to 0.50%, especially for the IA-GI / GA or IA / QT-GI / GA approach. Furthermore, the total aluminum and silicon addition is optimized to obtain 3 to 10% retained austenite. Preferably, the total aluminum and silicon content is in the range of 0.6 to 1.2%.

[0023] Chromium and Molybdenum: Cr 0.01% to 0.90%, Mo 0.01% to 0.50%. Chromium and molybdenum suppress the formation of ferrite and pearlite when cooled from the annealing temperature, improving hardenability and tensile strength. If the total of chromium and molybdenum exceeds 0.8%, cold rolling may become difficult.

[0024] Titanium: 0.01% to 0.10%. Titanium is added to protect boron in solid solution, resulting in the formation of TiN. Excess titanium, when added with molybdenum, also forms fine titanium carbides, which strengthen ferrite and tempered martensite.

[0025] Niobium: 0.01% or more and 0.04% or less. Niobium forms precipitates, which refines crystal grains and increases tensile strength.

[0026] Boron: 0.0005% or more and 0.005% or less. Boron can suppress the formation of ferrite during cooling from the annealing temperature. This helps prevent a decrease in tensile strength below 1480 MPa. In some examples, boron may be 0.0005% or more and 0.003% or less.

[0027] Sulfur: 0.01% or less. Sulfur combines with manganese to form MnS, an inclusion that can cause cracking and weldability problems.

[0028] Phosphorus: 0.05% or less. Excessive phosphorus causes grain boundary segregation, which leads to embrittlement.

[0029] The remainder of the steel composition consists of iron and unavoidable impurities resulting from melting. Table 1 shows some example steels having the compositional makeup and amounts in weight percent described above, along with some comparative steels.

[0030] [Table 1]

[0031] The following describes a method for producing steel sheets having the compositions listed in Table 1 above. In one example, an approximately 25 kg ingot was melted in air to have one of the preferred compositions described above. The molten material was then cast into an ingot, which was then hot rolled at 1260°C to a hot band having a gauge of 3.0 to 4.0 mm. The hot rolling finishing temperature was 850 to 900°C, and the coiling temperature was approximately 600°C. The hot band was then annealed and cold rolled at a reduction of 45 to 65% to a finished thickness of 1.2 to 2.0 mm. Compositions F and G were melted in a mill furnace and continuously cast into slabs weighing approximately 18,000 kg. The slab was then hot rolled at 1260°C to a finished hot band having a gauge of 3.0 to 4.0 mm. The finishing temperature of the hot rolling was 850 to 900 degrees Celsius, and the coiling temperature was about 600 degrees Celsius. Thereafter, the hot strip material was annealed and cold rolled at 45% reduction to be finished to a plate thickness of 1.8 mm to 2.0 mm.

[0032] Annealing simulations similar to the thermal profiles of hot-dip galvanizing / galvannealed electrogalvanizing plants were performed for each composition according to Figures 1-3. Tests performed on steels according to the present invention showed that, in the case of the FA / QT-GI / GA process, ultra-high-strength steels with ultimate tensile strengths of 1480-1750 MPa can be produced by performing a full austenitizing anneal in the temperature range of 840-860 °C (above Ac3), followed by quenching in the temperature range of 250-375 °C, followed by GI / GA and temper rolling. In the case of the IA-GI / GA or IA / QT-GI / GA approach, ultra-high-strength steels with ultimate tensile strengths of 1480-1750 MPa can be produced by performing an intercritical annealing in the temperature range of 750-830 °C (between Ac1 and Ac3), followed by cooling or quenching to a temperature below the galvanizing temperature, followed by GI / GA and temper rolling. To simulate the stretch-leveling that occurs in a factory annealing line, temper rolling with elongation of 0.15 to 0.25% was carried out. [Example]

[0033] Class A by FA / QT-GI / GA An exemplary steel with a composition of 0.200% C, 2.83% Mn, 0.620% Si, 0.018% Al, 0.130% V, 0.500% Cr, 0.110% Mo, 0.026% Ti, 0.021% Nb, 0.0025% B, 0.012% P, 0.0036% S, and 0.0071% N was melted, hot-rolled, annealed, and cold-rolled. Annealing simulation was performed using the FA / QT-GI / GA approach according to Figure 1. The soaking temperature was 840°C, and the quenching temperature ranged from 325 to 375°C. The tensile properties of this steel are shown in Table 2. The ultimate tensile strength (UTS) was 1480 to 1750 MPa, and the total elongation (TEL) was greater than 7.0%. Tensile properties were tested in the longitudinal direction. The structure has 25-65% tempered martensite, 70-35% fresh martensite, and 3-10% retained austenite.

[0034] [Table 2] [Example]

[0035] FA / QT-GI / GA Class B An exemplary steel with a composition of 0.206% C, 3.12% Mn, 0.630% Si, 0.033% Al, 0.150% V, 0.510% Cr, 0.090% Mo, 0.028% Ti, 0.020% Nb, 0.0023% B, 0.011% P, 0.0039% S, and 0.0047% N was melted, hot-rolled, annealed, and cold-rolled. Annealing simulation was performed using the FA / QT-GI / GA approach according to Figure 1. The soaking temperature was 840°C, and the quenching temperature ranged from 275 to 375°C. The tensile properties of this steel are shown in Table 3. The UTS was 1480 to 1750 MPa, and the TEL was 7.0% or higher. The tensile properties were tested in the longitudinal direction. The structure has 25-65% tempered martensite, 70-35% fresh martensite, and 3-10% retained austenite.

[0036] [Table 3] [Example]

[0037] Class C by FA / QT-GI / GA An exemplary steel with a composition of 0.224% C, 3.08% Mn, 0.670% Si, 0.031% Al, 0.140% V, 0.410% Cr, 0.090% Mo, 0.029% Ti, 0.020% Nb, 0.0024% B, 0.013% P, 0.0044% S, and 0.0057% N was melted, hot-rolled, annealed, and cold-rolled. Annealing simulation was performed using the FA / QT-GI / GA approach according to Figure 1. The soaking temperature was 840°C, and the quenching temperature ranged from 275 to 375°C. The tensile properties of this steel are shown in Table 4. The UTS was 1480 to 1750 MPa, and the TEL was 7.0% or higher. The tensile properties were tested in the longitudinal direction. The structure has 25-65% tempered martensite, 70-35% fresh martensite, and 3-10% retained austenite.

[0038] [Table 4] [Example]

[0039] Class B according to IA-GI / GA An exemplary steel with a composition of 0.206% C, 3.12% Mn, 0.630% Si, 0.033% Al, 0.150% V, 0.510% Cr, 0.090% Mo, 0.028% Ti, 0.020% Nb, 0.0023% B, 0.011% P, 0.0039% S, and 0.0047% N was melted, hot-rolled, annealed, and cold-rolled. Annealing simulations using the IA-GI / GA or IA / QT-GI / GA approach were performed according to Figures 2-3, with intercritical annealing soak temperatures ranging from 765 to 800 degrees Celsius. The tensile properties of this steel are shown in Table 5, with UTS values ​​of 1480 to 1750 MPa and TEL values ​​of 7.0% or greater. Tensile properties were tested in the longitudinal direction. The microstructure has 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

[0040] [Table 5] [Example]

[0041] Class C due to IA-GI / GA or IA / QT-GI / GA An exemplary steel with a composition of 0.224% C, 3.08% Mn, 0.670% Si, 0.031% Al, 0.140% V, 0.410% Cr, 0.090% Mo, 0.029% Ti, 0.020% Nb, 0.0024% B, 0.013% P, 0.0044% S, and 0.0057% N was melted, hot-rolled, annealed, and cold-rolled. Annealing simulations using the IA-GI / GA or IA / QT-GI / GA approach were performed according to Figures 2-3, with critical soak temperatures of 765-800°C. The tensile properties of this steel are shown in Table 6, with a UTS of 1480-1750 MPa and a TEL of 7.0% or greater. Tensile properties were tested in the longitudinal direction. The microstructure has 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

[0042] [Table 6] [Example]

[0043] FA / QT-GI / GA Class D An exemplary steel with a composition of 0.193% C, 2.77% Mn, 0.510% Si, 0.038% Al, 0.140% V, 0.490% Cr, 0.096% Mo, 0.028% Ti, 0.020% Nb, 0.0020% B, 0.012% P, 0.0046% S, and 0.0060% N was melted, hot-rolled, annealed, and cold-rolled. Annealing simulations using the FA / QT-GI / GA approach were performed according to Figure 1, with a soaking temperature of 840°C and quenching temperatures ranging from 345 to 360°C. The tensile properties of this steel are shown in Table 7, with a UTS of 1480 to 1750 MPa and a TEL of 7.0% or greater. Tensile properties were tested in the longitudinal direction. The structure has 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

[0044] [Table 7] [Example]

[0045] Class E by FA / QT-GI / GA An exemplary steel with a composition of 0.204% C, 2.87% Mn, 0.500% Si, 0.150% Al, 0.120% V, 0.540% Cr, 0.098% Mo, 0.031% Ti, 0.021% Nb, 0.0022% B, 0.003% P, 0.0058% S, and 0.0058% N was melted, hot-rolled, annealed, and cold-rolled. Annealing simulation was performed using the FA / QT-GI / GA approach according to Figure 1, with a soaking temperature of 840°C and a quenching temperature range of 320-375°C. The tensile properties of this steel are shown in Table 8, with a UTS of 1480-1750 MPa and a TEL of 7.0% or greater. Tensile properties were tested in the longitudinal direction. The structure has 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

[0046] [Table 8] [Example]

[0047] Class E due to IA-GI / GA or IA / QT-GI / GA An exemplary steel with a composition of 0.204% C, 2.87% Mn, 0.500% Si, 0.150% Al, 0.120% V, 0.540% Cr, 0.098% Mo, 0.031% Ti, 0.021% Nb, 0.0022% B, 0.003% P, 0.0007% S, and 0.0058% N was melted, hot-rolled, annealed, and cold-rolled. Annealing simulations using the IA-GI / GA or IA / QT-GI / GA approach were performed according to Figures 2-3, with critical soak temperatures ranging from 776 to 788°C. The tensile properties of this steel are shown in Table 9, with a UTS of 1480 to 1750 MPa and a TEL of 7.0% or greater. Tensile properties were tested in the longitudinal direction. The structure has 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

[0048] [Table 9] [Example]

[0049] Steel F (Mill Material Simulation) by FA / QT-GI / GA An exemplary steel with a composition of 0.195% C, 2.99% Mn, 0.610% Si, 0.040% Al, 0.130% V, 0.460% Cr, 0.100% Mo, 0.034% Ti, 0.024% Nb, 0.0015% B, 0.009% P, 0.0012% S, and 0.0046% N was melted and cast in a mill furnace, followed by hot rolling, annealing, and cold rolling. The steel annealing simulation using the FA / QT-GI / GA method was performed according to Figure 1, with a soaking temperature of 850-860°C and a quenching temperature of 340-346°C. The tensile properties of this steel are shown in Table 10, with a UTS of 1480-1750 MPa and a TEL of 7.0% or greater. Tensile properties were tested in the transverse direction. The structure has 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

[0050] [Table 10] [Example]

[0051] Steel F (Mill Material Simulation) using FA / QT-GI / GA method An exemplary steel with a composition of 0.206% C, 3.06% Mn, 0.690% Si, 0.041% Al, 0.140% V, 0.440% Cr, 0.103% Mo, 0.034% Ti, 0.023% Nb, 0.0015% B, 0.008% P, 0.0011% S, and 0.0056% N was melted and cast in a mill furnace, followed by hot rolling, annealing, and cold rolling. Annealing simulations using the FA / QT-GI / GA method were performed according to Figure 1, with soaking temperatures ranging from 850 to 860°C and quenching temperatures ranging from 340 to 346°C. The tensile properties of this steel are shown in Table 11, with a UTS of 1480 to 1750 MPa and a TEL of 7.0% or greater. Tensile properties were tested in the transverse direction. The structure has 15-30% ferrite, 80-65% martensite, and 3-10% retained austenite.

[0052] [Table 11] [Example]

[0053] Comparison with FA / QT-GI / GA A comparative steel with a composition of 0.199% C, 2.83% Mn, 0.600% Si, 0.029% Al, 0.003% V, 0.001% Cr, 0.003% Mo, 0.027% Ti, 0.010% Nb, 0.0022% B, 0.012% P, 0.0036% S, and 0.0069% N was produced, hot-rolled, annealed, and cold-rolled. Annealing simulation was performed using the FA / QT-GI / GA method according to Figure 1. The soaking temperature was 840°C, and the quenching temperature was in the range of 250-350°C. This comparative steel, Q, had the following tensile properties, as shown in Table C1. The UTS was 990-1200 MPa, which is significantly lower than the UTS of 1480-1750 MPa reported in Examples 1-11. The poor tensile properties result from a non-optimal composition, such as a lack of chromium and molybdenum strengthening.

[0054] [Table C1] [Example]

[0055] Comparison with FA / QT-GI / GA A comparative steel with a composition of 0.200% C, 2.74% Mn, 0.610% Si, 0.028% Al, 0.003% V, 0.520% Cr, 0.096% Mo, 0.026% Ti, 0.020% Nb, 0.0022% B, 0.012% P, 0.0034% S, and 0.0072% N was produced and hot-rolled, annealed, and cold-rolled. Annealing simulation using the FA / QT-GI / GA method was performed according to Figure 1. The soaking temperature was 840°C and the quenching temperature was in the range of 250-350°C. This comparative steel, R, had the following tensile properties as shown in Table C2: UTS of 1140-1285 MPa, which is significantly lower than the UTS of 1480-1750 MPa reported in Examples 1-11. The poor tensile properties result from a non-optimal composition, such as the lack of vanadium carbide precipitation strengthening.

[0056] [Table C2]

[0057] It should be understood that any one or more of the teachings, expressions, embodiments, examples, etc. described herein may be combined with any one or more of the other teachings, expressions, embodiments, examples, etc. described herein. Accordingly, the teachings, expressions, embodiments, examples, etc. described below should not be viewed in isolation from one another. Various suitable ways in which the teachings herein can be combined will be readily apparent to those skilled in the art in light of the teachings herein. Such modifications and variations are intended to be within the scope of the claims.

[0058] While various embodiments of the present invention have been shown and described, further adaptations of the methods and systems described herein may be achieved by appropriate modifications by those skilled in the art without departing from the scope of the present invention. While some such potential modifications have been mentioned, others will be apparent to those skilled in the art. For example, the examples, embodiments, geometries, materials, dimensions, proportions, steps, etc. described above are illustrative and not required. Accordingly, the scope of the present invention should be considered in light of the following claims and is understood not to be limited to the details of structure and operation shown and described in the specification and drawings.

Claims

1. 1. A steel having, in weight percent, 0.15-0.26% C, 2.10-3.60% Mn, 0.05-0.85% Si, 0.001-0.85% Al, 0.01-0.90% Cr, 0.01-0.50% Mo, 0.01-0.10% Ti, 0.01-0.04% Nb, 0.01-0.30% V, 0.0001-0.0005% B, less than 0.01% N, less than 0.01% S, less than 0.005% P, and the balance Fe and impurities.

2. The steel of claim 1 wherein the steel is formed into a sheet.

3. 3. A steel according to claim 1 or claim 2, wherein the steel is an ultra-high strength steel.

4. 4. The steel according to claim 1, wherein the ultimate tensile strength is greater than or equal to about 1480 MPa and less than or equal to about 1750 MPa.

5. 5. The steel according to claim 1, wherein the yield strength is greater than or equal to about 1050 MPa and less than or equal to about 1050 MPa.

6. A steel according to any one of claims 1 to 5, wherein the total elongation is greater than or equal to about 7%.

7. 7. The steel of claim 1, wherein the microstructure of the steel has, by area percent, about 10-25% ferrite, about 75-90% martensite, and about 3-10% retained austenite.

8. A steel according to claim 7, wherein the ferrite content is preferably between 13 and 19%.

9. A steel according to any one of claims 7 or 8, wherein the martensite is preferably between 75 and 80%.

10. A steel according to any one of claims 7 to 9, wherein the retained austenite is preferably between 5 and 8%.

11. 7. The steel of claim 1, wherein the microstructure of the steel has, by area percent, about 20-65% tempered martensite, about 35-80% fresh martensite, and about 3-10% retained austenite.

12. A steel according to claim 11, wherein the tempered martensite is preferably between 35 and 50%.

13. A steel according to any one of claims 11 or 12, wherein the fresh martensite is preferably between 50 and 65%.

14. A steel according to any one of claims 11 to 13, wherein the retained austenite is preferably between 5 and 8%.

15. 9. The steel of any one of claims 1 to 8, wherein the overall microstructure of the steel comprises ferrite and overaged martensite, and the ferrite and overaged martensite are strengthened by one or more of: (a) solid solution strengthening; (b) precipitation of vanadium carbides and / or titanium niobium carbides; (c) grain refinement strengthening; and (d) strain strengthening or work hardening.

16. A steel according to any one of the preceding claims, wherein the steel is adapted for use in galvanizing and electrogalvannealing processes.

17. The steel according to any one of claims 1 to 16, having 0.0001 to 0.0003% B.

18. 1. A method of producing steel, comprising: (a) melting an ingot having a composition of 0.15-0.26% C, 2.10-3.60% Mn, 0.05-0.85% Si, 0.001-0.85% Al, 0.01-0.90% Cr, 0.01-0.50% Mo, 0.01-0.10% Ti, 0.01-0.04% Nb, 0.01-0.30% V, 0.0001-0.0005% B, less than 0.01% N, less than 0.01% S, less than 0.005% P, and the balance being Fe and impurities; (b) hot rolling the ingot into a hot band having a gauge of about 3.0-4.0 mm; (c) annealing the hot band; (d) cold rolling the annealed hot band to a reduced gauge to form a plate of about 1.2 to 2.0 mm gauge; A method comprising:

19. 19. The method of claim 18, wherein the steel is fully annealed at a temperature range of about 840-860 degrees Celsius and above the Ac3 temperature, then quenched to a temperature range of about 250-375 degrees Celsius, galvanized and electrogalvannealed, and then temper rolled.

20. 19. The method of claim 18, wherein the steel is intercritical annealed in the temperature range of about 750-830 degrees Celsius between the Ac1 temperature and the AC3 temperature, then cooled below the galvanizing temperature, galvanized and electrogalvannealed, and then temper rolled.

21. 19. The method of claim 18, wherein the steel is intercritical annealed between an Ac1 temperature and an AC3 temperature in the temperature range of about 750-830 degrees Celsius, then cooled to below the galvanizing temperature, galvanized, and then temper rolled.

22. 19. The method of claim 18, having 0.0001 to 0.0003% B.

23. 20. The method of claim 18, wherein the step of cold rolling the annealed hot band to reduce the gauge forms a plate of about 1.2 to 1.6 mm.

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