High-strength galvanized and galvanized annealed steel sheet and method of manufacture

Abstract

The present invention relates to an ultra-high strength steel which improves the treatment window of galvanization and galvanization annealing through a novel alloying concept. In addition to the alloying concept, the steel also includes a strengthened ferrite and over-aged martensite microstructure, and the volume fraction of the magnetic phase is controlled. The steel may be treated in the form of a sheet and galvanized and galvanized annealed.

Description

[0001] priority

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 451,994, filed March 14, 2023, entitled "High Strength Galvanized and Galvannealed Steel Sheets and Manufacturing Method," the disclosure of which is incorporated herein by reference. Background Technology

[0003] Automotive applications demand high-strength and high-ductility steels. The use of high-strength steels has led to advancements in vehicle weight reduction and improvements affecting safety. At least in the automotive steel market, new steels are designed to meet the demands for high ultimate tensile strength, good ductility, improved strain hardening behavior and formability, and the ability to be galvanized (GI) and galvanized annealed (GA).

[0004] Several groups of steels offering different strength values ​​have been proposed and are used in the automotive and other markets. One example is dual-phase (DP) steel, which includes a ferrite phase and an island-integrated martensite phase. DP steel offers excellent ductility and formability, as well as relatively low yield strength (YS) and ultimate tensile strength (UTS) (i.e., typically less than 600 MPa). Another example is multiphase (MP) steel, which includes a ferrite phase, a bainite phase, and a martensite phase. Compared to DP steel, MP steel offers higher yield strength and ultimate tensile strength, typically in the range of 700 to 1000 MPa. Yet another example is multiphase (CP) steel, which contains a ferrite-bainite phase, a martensite phase, and retained austenite and / or pearlite. Compared to DP and MP steels, CP steel offers higher strength, typically in the range of 980 to 1200 MPa. There are also single-phase (SP) steels containing microstructures with bainite or martensite, which offer extremely high yield strength and ultimate tensile strength (i.e., ultimate tensile strength in the range of 1300 to 1700 MPa), but have relatively low ductility and formability compared to DP, MP and CP steels.

[0005] By way of example only, U.S. Patent Publication US2013 / 008570 relates to high-strength steel having an ultimate tensile strength of 1100 MPa, formability, strength-tensile balance, and flexural workability. The microstructure described in US2013 / 008570 consists of 50% or more martensite, 15% or more ferrite-bainite, and 0 to 5% polygonal ferrite. This publication does not disclose the steel's ability to undergo galvanizing-galvanizing annealing.

[0006] International Patent Publication WO2012 / 153016 relates to cold-rolled steel with an ultimate tensile strength exceeding 1000 MPa and an elongation exceeding 12%. The microstructure described in WO2012 / 153016 consists of 5 to 15% martensite, 10 to 15% retained austenite, and 5 to 20% polygonal ferrite. This publication does not disclose the steel's ability to undergo galvanizing-galvanizing annealing.

[0007] U.S. Patent 11,047,020 relates to cold-rolled and hot-dip-soaked steels having an ultimate tensile strength of 980 to 1180 MPa. The described steel microstructure comprises 50 to 90% martensite and 5 to 50% ferrite + bainite. U.S. Patent 11,047,020 discloses steels capable of galvanizing-galvanizing annealing; however, the maximum ultimate tensile strength is 1180 MPa.

[0008] U.S. Patent 8,840,834 relates to ultra-high strength steels with an ultimate tensile strength of 1400 MPa or higher. The microstructure comprises 80% or more self-tempered martensite with precipitated iron-based carbides, less than 5% ferrite, 10% or less bainite, and 5% or less retained austenite. However, the steel of the '834 patent will exhibit a loss of galvanizing (GA) capability due to the low percentage of magnetic phases (e.g., ferrite) present at elevated GA temperatures. Furthermore, if the magnetic phase increases, this will result in a decrease in tensile strength in the steel of the '834 patent at the expense of a higher percentage of self-tempered martensite.

[0009] Japanese Patent JP2528387 relates to ultra-high strength cold-rolled steel with an ultimate tensile strength of at least 1500 MPa and good formability, wherein annealing is performed under certain conditions, rapid cooling with spray water, and over-aging treatment are used. The steel of this Japanese patent would be unsuitable for galvanizing annealing because the galvanizing annealing temperature is much higher than the over-aging temperature used.

[0010] International patent publication WO2021 / 176249 relates to ultra-high strength cold-rolled and galvanized annealed steel sheets with a tensile strength greater than or equal to 1450 MPa. The steel microstructure consists of 80 to 90% martensite with the remainder being ferrite and bainite (5% or more ferrite and / or 5% or more bainite). Intercritical annealing (IA) between Ac1 and Ac3 temperatures is used to obtain 5 to 15% ferrite, thus achieving GA capability. However, the steel described in WO2021 / 176249 provides an extremely narrow IA treatment window where the ferrite percentage is difficult to control. At lower IA temperatures, GA capability is present, but the ultimate tensile strength is below 1450 MPa. At higher IA temperatures, the ultimate tensile strength is above 1450 MPa, but GA capability is lost. This publication fails to consider the effect of the IA treatment window on the balance between tensile strength and GA capability.

[0011] Despite the availability of various steels (some of which have been described above), there remains a need for improved galvanized-galvanized annealed steel products with ultra-high strength (greater than 1480 MPa) and good ductility, featuring a wider GI / GA treatment window to address the balance between tensile strength and GA capacity. Meeting this need will improve manufacturing reliability, especially in facilities employing induction heating. Although various steels and methods for preparing steel have been made and used, it is considered that no one had made or used the invention described herein before the inventor. Attached Figure Description

[0012] Although this specification concludes with claims that particularly point out and explicitly assert the invention, it is believed that the invention can be better understood from the following description of certain examples taken in conjunction with the accompanying drawings, wherein similar reference numerals indicate the same elements.

[0013] Figure 1 This is a schematic diagram of a fully austenitic process and a quenching and tempering (FA / QT-GI / GA) method, which uses galvanizing and galvanizing annealing at elevated temperatures to achieve 20 to 65% martensite and tempered martensite.

[0014] Figure 2 This is a schematic diagram of an intercritical annealing (IA-GI / GA) process, which has an intercritical annealing treatment window between Ac3 and Ac1 to obtain 10 to 25% ferrite, followed by galvanizing and galvanizing annealing.

[0015] Figure 3 This is a schematic diagram of an intercritical annealing and quenching and tempering (IA / QT-GI / GA) process, wherein the process has an intercritical annealing process window and then a quenching and tempering treatment window with galvanizing and galvanizing annealing.

[0016] Figure 4This is a schematic illustration of the intercritical annealing window, showing a non-optimized composition with an extremely narrow intercritical annealing window and an optimized composition with an intercritical annealing window that is two to three times wider.

[0017] The illustrations are not intended to be limiting in any way, and it is conceivable that various embodiments of the invention may be implemented in a variety of other ways (including those not necessarily shown in the illustrations). The accompanying drawings, which are incorporated herein and form a part of this specification, illustrate several aspects of the invention and, together with this description, serve to explain the principles of the invention; however, it should be understood that the invention is not limited to the precise configuration shown. Detailed Implementation

[0018] The following description of certain examples of the invention is not intended to limit the scope of the invention. Other examples, features, aspects, embodiments, and advantages of the invention will become apparent to those skilled in the art from the following description, which is made by way of illustration and is considered as one of the best modes of implementation. As will be appreciated, the invention can have other different and obvious aspects, all of which do not depart from the invention. Therefore, the drawings and descriptions are to be regarded as illustrative in nature and not restrictive.

[0019] As explained above, in conventional steelmaking, there is a trade-off between high strength and galvanized annealing capability (GA capability), where strength must be sacrificed to achieve high GA capability. Certain embodiments of the present invention aim to resolve this trade-off by using a novel alloy concept for ultra-high strength steel sheets with a minimum tensile strength of 1480 to 1750 MPa and by using novel methods for manufacturing such ultra-high strength galvanized (GI) and / or galvanized annealed (GA) steels. This new alloy concept and manufacturing method broaden the GI / GA treatment window to achieve a good combination of high strength and GA capability.

[0020] Certain embodiments of the present invention achieve a combination of yield strength, tensile strength, ductility, and formability that provides a wide GI / GA processing window and good manufacturing reliability, which is not typically achievable through conventional steelmaking. In some instances, the steels obtained from the compositions and methods disclosed herein have tensile strengths of 1480 to 1750 MPa, yield strengths of 1050 to 1280 MPa, and total elongation greater than or equal to 7%. In this regard, embodiments of the present disclosure relate to an ultra-high strength galvanized and / or galvanized annealed steel sheet with high tensile and yield strengths, as well as good ductility, formability, and galvanizing and annealing capabilities. In one example, the alloy composition, by weight percentage, is as follows: C 0.15 to 0.26%, Mn 2.10 to 3.60%, Si 0.05 to 0.85%, Al 0.001 to 0.85%, Cr 0.01 to 0.90%, Mo 0.01 to 0.50%, Ti 0.01 to 0.10%, Nb 0.01 to 0.04%, V 0.01 to 0.30%, B 0.0001 to 0.005%, N less than 0.01%, S less than 0.01%, P less than 0.05%. The remainder is Fe and incidental impurities.

[0021] Three exemplary manufacturing methods are applicable to the alloy concept, and these methods involve (1) fully austenitic steel with quenching and tempering (FA / QT-GI / GA), (2) intercritical annealing (IA-GI / GA), and (3) intercritical annealing, quenching, and tempering (IA / QT-GI / GA). With IA-GI / GA and IA / QT-GI / GA, the steel microstructure comprises (by area) 10 to 25% ferrite, 75 to 90% martensite, and 3 to 10% retained austenite. With FA-GI / GA, the steel microstructure comprises (by area) 20 to 65% tempered martensite, 35 to 80% newly prepared martensite, and 3 to 10% retained austenite to achieve a good combination of high strength and good galvanizing annealing capability.

[0022] To achieve good GA (Growth Attenuation) capability in ultra-high strength steels, a sufficient amount of magnetic phase in the microstructure (e.g., ferrite and / or martensite) is required at elevated temperatures to ensure the GA process. A higher percentage of magnetic phase results in better GA capability. However, ferrite is a soft phase, and tempered and over-aged martensite is not a hard phase either. To achieve ultra-high strength, a higher percentage of hard phase is required. This can be achieved by increasing the strength of the ferrite and over-aged martensite phases. In at least some of the embodiments described herein, the available strengthening mechanisms include at least one or more of the following: (A) solid solution strengthening of Mn, Si, Al, and Mo, etc.; (B) precipitation strengthening of vanadium carbide, titanium carbide, and / or niobium carbide; (C) grain refinement strengthening; and / or (D) strain strengthening by tempered rolling and / or stretching.

[0023] In addition to strengthening all phases, the volume fraction of the softer phase (magnetic phase) is controlled to optimize GA capability and ultra-high strength within a wide QT or IA processing window. In the FA / QT-GI / GA approach, the microstructure is designed as follows: 20 to 65%, preferably 35 to 50%, of QT-tempered martensite for robust GA capability; 35 to 80%, preferably 65 to 50%, of newly prepared martensite for ultra-high strength; and 3 to 10%, preferably 5 to 8%, of retained austenite for strain strengthening and good ductility. In the IA-GI / GA and IA / QT-GI / GA approaches, the microstructure is designed as follows: 10 to 25%, preferably 13 to 19%, of ferrite for robust GA capability; 75 to 90%, preferably 80 to 75%, of martensite for ultra-high strength; and 3 to 10%, preferably 5 to 8%, of retained austenite for strain strengthening and good ductility.

[0024] The following paragraphs explain at least some exemplary reasons for choosing the composition and range of the alloying concept described above. In some cases, the term "about" may be used when describing the range or value of the constituent elements. In this case, the term "about" should be given the broadest meaning based on the understanding of a person skilled in the art, or, in any case, should be given a specified value of + / - 10% if the broadest meaning based on the understanding of a person skilled in the art is unknown or uncertain.

[0025] Carbon: 0.15% or more and 0.26% or less. Carbon is an important element for strengthening martensite. If the carbon content is below 0.15%, the minimum required tensile strength of 1480 MPa cannot be achieved. If the carbon content is above 0.26%, it results in lower ductility and poorer weldability. In addition, a higher carbon content leads to a very low martensite onset (Ms) temperature, resulting in a narrow QT window. Preferably, the carbon content is in the range of 0.18% to 0.22%.

[0026] Manganese: 2.10% or more and 3.60% or less. Manganese is an element that strengthens martensite and solid solution-strengthened ferrite. To achieve a tensile strength of 1480 to 1750 MPa, the required amount of hard phase needs to be ensured. If the manganese content is less than 2.10%, the tensile strength will be less than 1480 MPa. If the manganese content is greater than 3.60%, it can cause Mn isolation, negatively impacting tensile strength. Preferably, the manganese content is in the range of 2.60% to 3.30%.

[0027] Silicon: 0.05% or more and 0.85% or less. Silicon increases strength through solid solution strengthening and also acts as a ferrite stabilizer to enhance the Ac3 temperature. Excess silicon reduces hot processability. Furthermore, the coating ability of hot dip coating can be impaired due to the formation of silicon oxide on the surface. In IA-GI / GA or IA / QT-GI / GA processes, the addition of silicon contents should be optimized by adding aluminum to ensure a wide IA processing window. Preferably, the silicon content is in the range of 0.15% to 0.60%.

[0028] Aluminum: 0.001% or more and 0.80% or less. Aluminum forms AlN to avoid the formation of boron nitride, and it is also a strong ferrite stabilizer for significantly increasing the Ac3 temperature. Excess aluminum will result in extremely high annealing temperatures, causing manufacturability problems. Therefore, the aluminum content is optimized together with the silicon content to provide a wide IA processing window. Preferably, especially for IA-GI / GA or IA / QT-GI / GA processes, the aluminum content is in the range of 0.15% to 0.50%. Furthermore, the total addition of aluminum and silicon is optimized to obtain 3% to 10% retained austenite. Preferably, the total aluminum + silicon content is in the range of 0.6% to 1.2%.

[0029] Chromium and molybdenum: Cr 0.01% or more and 0.90% or less; Mo 0.01% or more and 0.50% or less. Chromium and molybdenum inhibit the formation of ferrite and pearlite during cooling from the annealing temperature and improve hardening properties and tensile strength. If the total chromium + molybdenum content exceeds 0.8%, it can cause problems with cold rolling.

[0030] Vanadium: 0.01% or more and 0.30% or less. Vanadium is a carbide-forming element. It strengthens ferrite and tempered martensite through precipitation strengthening. Preferably, the vanadium content is in the range of 0.030 to 0.150%.

[0031] Titanium: 0.01% or more and 0.10% or less. Titanium is added to form TiN, thereby protecting boron in the solid solution. Additionally, when molybdenum is added, excess titanium also forms micro-titanium carbides, thereby strengthening the ferrite and tempered martensite.

[0032] Niobium: 0.01% or more and 0.04% or less. Niobium can form precipitates and has a grain-refining effect to increase tensile strength.

[0033] Boron: 0.0005% or more and 0.005% or less. Boron can inhibit ferrite formation during cooling from the annealing temperature. Therefore, it helps prevent the tensile strength from dropping below 1480 MPa. In some instances, boron may be 0.0005% or more and 0.003% or less.

[0034] Sulfur: 0.01% or less. Sulfur combines with manganese to form MnS, which is an inclusion that causes cracking and weldability problems.

[0035] Phosphorus: 0.05% or less. Excess phosphorus causes grain boundary isolation, leading to embrittlement.

[0036] The remainder of the steel composition includes iron and unavoidable impurities from the melting process. Table 1 illustrates several examples of steels with the composition and weight percentages discussed above, as well as some comparative steels.

[0037] Table 1

[0038]

[0039] The following describes a method for manufacturing a steel sheet with the composition described in Table 1 above. In one example, approximately 25 kg of ingot is melted in air to have one of the preferred compositions described above. The ingot is then formed from the molten material by casting and hot-rolled at 1260°C into a strip with a gauge of 3.0 to 4.0 mm. The final hot-rolling temperature is 850 to 900°C, and the coiling temperature is approximately 600°C. The strip is then annealed and cold-rolled and reduced by 45 to 65% until a sheet with a gauge of 1.2 mm to 2.0 mm is formed. Compositions F and G are melted in a grinding furnace and continuously cast into a plate of approximately 18,000 kg. The plate is hot-rolled at 1260°C into a strip with a gauge of 3.0 to 4.0 mm. The final hot-rolling temperature is 850 to 900°C, and the coiling temperature is approximately 600°C. The hot strip is then annealed and cold-rolled and reduced by 45% until a sheet with a gauge length of 1.8 mm to 2.0 mm is obtained.

[0040] Similar to according to Figures 1 to 3 The hot-dip galvanizing / galvanizing annealing equipment thermal profiles were used to perform annealing simulations for each composition. Tests conducted with the steel according to the invention have shown that, in the FA / QT-GI / GA configuration, ultra-high strength steel with an ultimate tensile strength of 1480 to 1750 MPa can be produced by performing full austenitizing annealing in a temperature range of 840 to 860 °C (above Ac3), followed by quenching in a temperature range of 250 to 375 °C, and then performing GI / GA and tempering rolling. In the IA-GI / GA or IA / QT-GI / GA configuration, ultra-high strength steel with an ultimate tensile strength of 1480 to 1750 MPa can be produced by performing intercritical annealing in a temperature range of 750 to 830 °C (between Ac1 and Ac3), followed by cooling or quenching to a temperature below the galvanizing temperature, and then performing GI / GA and tempering rolling. Tempering rolling with an elongation of 0.15 to 0.25% is performed in the equipment annealing line to simulate stretch leveling.

[0041] Example

[0042] Example 1 – Using FA / QT-GI / GA steel A

[0043] An exemplary steel with the 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. According to... Figure 1 Annealing simulations were performed using the FA / QT-GI / GA method, with an immersion temperature of 840℃ and a quenching temperature ranging from 325 to 375℃. As shown in Table 2, this steel exhibits the following tensile properties: ultimate tensile strength (UTS) of 1480 to 1750 MPa and total elongation (TEL) greater than 7.0%. Tensile properties were tested in the longitudinal direction. The microstructure comprises 25 to 65% tempered martensite, 70 to 35% newly prepared martensite, and 3 to 10% retained austenite.

[0044] Table 2

[0045] Properties of steel A and FA / QT-GI / GA method

[0046] Immersion temperature, ℃ Quenching temperature, °C Tempering rolling UTS, MPa YS, MPa TEL, % Remark 840 325 no 1514 963 10.0 FA / QT 840 325 no 1522 985 8.7 FA / QT 840 350 no 1543 1037 7.6 FA / QT 840 350 yes 1603 1175 8.7 FA / QT 840 350 yes 1599 1164 10.0 FA / QT 840 375 no 1569 1062 8.2 FA / QT 840 375 no 1572 1051 8.8 FA / QT

[0047] Example 2 – Using FA / QT-GI / GA steel B

[0048] An exemplary steel with the 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. According to... Figure 1 Annealing simulations were performed using the FA / QT-GI / GA method, with an immersion temperature of 840℃ and a quenching temperature ranging from 275 to 375℃. As shown in Table 3, this steel exhibits the following tensile properties: UTS of 1480 to 1750 MPa and TEL greater than 7.0%. Tensile properties were tested in the longitudinal direction. The microstructure comprises 25 to 65% tempered martensite, 70 to 35% newly prepared martensite, and 3 to 10% retained austenite.

[0049] Table 3

[0050] Properties of steel B and FA / QT-GI / GA method

[0051]

[0052]

[0053] Example 3 – Using FA / QT-GI / GA steel C

[0054] An exemplary steel with the 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. According to... Figure 1 Annealing simulations were performed using the FA / QT-GI / GA method, with an immersion temperature of 840℃ and a quenching temperature ranging from 275 to 375℃. As shown in Table 4, this steel exhibits the following tensile properties: UTS of 1480 to 1750 MPa and TEL greater than 7.0%. Tensile properties were tested in the longitudinal direction. The microstructure comprises 25 to 65% tempered martensite, 70 to 35% newly prepared martensite, and 3 to 10% retained austenite.

[0055] Table 4

[0056] Properties of steel C and FA / QT-GI / GA method

[0057] Immersion temperature, ℃ Quenching temperature, °C Tempering rolling UTS, MPa YS, MPa TEL, % Remark 840 275 no 1542 891 9.1 FA / QT 840 275 yes 1557 1037 9.0 FA / QT 840 300 no 1648 1026 8.1 FA / QT 840 300 yes 1685 1221 7.8 FA / QT 840 325 no 1659 1081 9.9 FA / QT 840 325 no 1669 1099 7.9 FA / QT 840 350 no 1669 1107 8.6 FA / QT 840 350 yes 1722 1229 8.9 FA / QT 840 375 no 1694 1133 9.1 FA / QT 840 375 no 1621 1070 8.9 FA / QT

[0058] Example 4 – Steel B using IA-GI / GA or IA / QT-GI / GA

[0059] An exemplary steel with the 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. According to... Figures 2 to 3 Annealing simulations were performed using IA-GI / GA or IA / QT-GI / GA methods, with critical intercalation immersion temperatures ranging from 765 to 800 °C. As shown in Table 5, this steel exhibits the following tensile properties: UTS of 1480 to 1750 MPa and TEL greater than 7.0%. Tensile properties were tested in the longitudinal direction. The microstructure comprises 15 to 30% ferrite, 80 to 65% martensite, and 3 to 10% retained austenite.

[0060] Table 5

[0061] Properties of steel B and IA-GI / GA or IA / QT-GI / GA method

[0062] Immersion temperature, ℃ Quenching temperature, °C Tempering rolling UTS, MPa YS, MPa TEL, % Remark 800 325 no 1591 997 8.9 IA / QT 800 325 no 1584 973 9.0 IA / QT 800 350 no 1580 963 9.6 IA / QT 800 350 no 1560 944 9.3 IA / QT 800 350 yes 1595 1125 8.5 IA / QT 800 350 yes 1594 1258 8.0 IA / QT 800 375 no 1599 1021 8.1 IA / QT 800 375 no 1605 1008 9.1 IA / QT 765 450 no 1506 797 7.0 IA 765 450 yes 1557 1145 7.0 IA

[0063] Example 5 – Steel C using IA-GI / GA or IA / QT-GI / GA

[0064] An exemplary steel with the 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. According to... Figures 2 to 3 Annealing simulations were performed using IA-GI / GA or IA / QT-GI / GA methods, with critical immersion temperatures ranging from 765 to 800 °C. As shown in Table 6, this steel exhibits the following tensile properties: UTS of 1480 to 1750 MPa and TEL greater than 7.0%. Tensile properties were tested in the longitudinal direction. The microstructure comprises 15 to 30% ferrite, 80 to 65% martensite, and 3 to 10% retained austenite.

[0065] Table 6

[0066] Properties of steel C and IA-GI / GA or IA / QT-GI / GA method

[0067]

[0068]

[0069] Example 6 – Steel D using FA / QT-GI / GA

[0070] An exemplary steel with the 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. According to... Figure 1 Annealing simulations were performed using the FA / QT-GI / GA method, with an immersion temperature of 840°C and a quenching temperature ranging from 345°C to 360°C. As shown in Table 7, this steel exhibits the following tensile properties: UTS of 1480 to 1750 MPa and TEL greater than 7.0%. Tensile properties were tested in the longitudinal direction. The microstructure consists of 15 to 30% ferrite, 80 to 65% martensite, and 3 to 10% retained austenite.

[0071] Table 7

[0072] Properties of steel D and FA / QT-GI / GA method

[0073] Immersion temperature, ℃ Quenching temperature, °C Tempering rolling UTS, MPa YS, MPa TEL, % Remark 840 345 no 1478 962 8.5 FA / QT 840 345 yes 1505 1106 8.4 FA / QT 840 360 no 1527 1030 7.8 FA / QT 840 360 yes 1541 1153 7.5 FA / QT

[0074] Example 7 – Steel E using FA / QT-GI / GA

[0075] An exemplary steel with the 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. According to... Figure 1 Annealing simulations were performed using the FA / QT-GI / GA method, with an immersion temperature of 840°C and a quenching temperature ranging from 320°C to 375°C. As shown in Table 8, this steel exhibits the following tensile properties: UTS of 1480 to 1750 MPa and TEL greater than 7.0%. Tensile properties were tested in the longitudinal direction. The microstructure consists of 15 to 30% ferrite, 80 to 65% martensite, and 3 to 10% retained austenite.

[0076] Table 8

[0077] Properties of steel E and FA / QT-GI / GA method

[0078] Immersion temperature, ℃ Quenching temperature, °C Tempering rolling UTS, MPa YS, MPa TEL, % Remark 840 320 no 1517 980 8.0 FA / QT 840 320 yes 1545 1139 8.0 FA / QT 840 330 no 1574 1011 8.7 FA / QT 840 330 yes 1599 1187 9.1 FA / QT 840 345 no 1578 1051 7.6 FA / QT 840 345 yes 1597 1218 7.9 FA / QT 840 360 no 1579 1060 8.9 FA / QT 840 360 yes 1598 1205 8.3 FA / QT 840 375 no 1577 1060 8.0 FA / QT 840 375 yes 1577 1172 9.3 FA / QT

[0079] Example 8 – Steel E using IA-GI / GA or IA / QT-GI / GA

[0080] An exemplary steel with the 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. According to... Figures 2 to 3 Annealing simulations were performed using IA-GI / GA or IA / QT-GI / GA methods, with critical immersion temperatures ranging from 776 to 788 °C. As shown in Table 9, this steel exhibits the following tensile properties: UTS of 1480 to 1750 MPa and TEL greater than 7.0%. Tensile properties were tested in the longitudinal direction. The microstructure consists of 15 to 30% ferrite, 80 to 65% martensite, and 3 to 10% retained austenite.

[0081] Table 9

[0082] Properties of steel E and IA-GI / GA or IA / QT-GI / GA method

[0083] Immersion temperature, ℃ Quenching temperature, °C Tempering rolling UTS, MPa YS, MPa TEL, % Remark 776 400 no 1515 780 8.7 IA / QT 776 400 yes 1531 950 7.8 IA / QT 780 400 no 1522 760 7.7 IA / QT 780 400 yes 1540 999 7.7 IA / QT 784 400 no 1510 780 7.7 IA / QT 784 400 yes 1540 983 7.2 IA / QT 788 400 no 1531 840 7.8 IA / QT 788 400 yes 1550 999 9.2 IA / QT

[0084] Example 9 – Steel F using FA / QT-GI / GA (Abrasive Material Simulation)

[0085] An exemplary steel with the 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 grinding furnace, followed by hot rolling, annealing, and cold rolling. According to... Figure 1 Annealing simulations were performed using the FA / QT-GI / GA method, with immersion temperatures ranging from 850 to 860°C and quenching temperatures ranging from 340 to 346°C. As shown in Table 10, this steel exhibits the following tensile properties: UTS of 1480 to 1750 MPa and TEL greater than 7.0%. Tensile properties were tested in the transverse direction. The microstructure consists of 15 to 30% ferrite, 80 to 65% martensite, and 3 to 10% retained austenite.

[0086] Table 10

[0087] Properties of steel F and FA / QT-GI / GA method (grinding material simulation)

[0088] Immersion temperature, ℃ Quenching temperature, °C Tempering rolling UTS, MPa YS, MPa TEL, % Remark 850 340 no 1491 937 8.2 FA / QT 850 340 yes 1518 1144 8.0 FA / QT 850 346 no 1458 914 7.5 FA / QT 850 346 yes 1467 1099 7.7 FA / QT 860 345 no 1491 941 8.3 FA / QT 860 345 yes 1516 1151 7.3 FA / QT 860 360 no 1471 929 8.6 FA / QT 860 360 yes 1537 1183 7.5 FA / QT

[0089] Example 10 – Steel G using FA / QT-GI / GA (Abrasive Material Simulation)

[0090] An exemplary steel with the 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 is melted and then hot-rolled, annealed, and cold-rolled in a grinding furnace. According to... Figure 1 Annealing simulations were performed using the FA / QT-GI / GA method, with immersion temperatures ranging from 850 to 860°C and quenching temperatures ranging from 340 to 346°C. As shown in Table 11, this steel exhibits the following tensile properties: UTS of 1480 to 1750 MPa and TEL greater than 7.0%. Tensile properties were tested in the transverse direction. The microstructure consists of 15 to 30% ferrite, 80 to 65% martensite, and 3 to 10% retained austenite.

[0091] Table 11

[0092] Properties of steel G and FA / QT-GI / GA method (grinding material simulation)

[0093]

[0094]

[0095] Example C1 – Comparative steel Q using FA / QT-GI / GA

[0096] A comparative steel with the 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 melted, hot-rolled, annealed, and cold-rolled. According to... Figure 1 Annealing simulations were performed using the FA / QT-GI / GA method, with an immersion temperature of 840°C and a quenching temperature ranging from 250 to 350°C. As shown in Table C1, this comparative steel Q exhibits the following tensile properties: UTS of 990 to 1200 MPa, significantly lower than the 1480 to 1750 MPa UTS described in Examples 1 to 11. The lower tensile properties are attributed to the suboptimal composition, such as the lack of chromium and molybdenum reinforcement.

[0097] Table C1

[0098] Properties of steel Q and FA / QT-GI / GA method

[0099] Immersion temperature, ℃ Quenching temperature, °C Tempering rolling UTS, MPa YS, MPa TEL, % Remark 840 250 no 993 829 11.8 FA / QT 840 300 no 1165 758 9.9 FA / QT 840 350 no 1193 744 11.3 FA / QT

[0100] Example C2 – Comparative steel R using FA / QT-GI / GA

[0101] A comparative steel with the 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 melted, hot-rolled, annealed, and cold-rolled. According to... Figure 1 Annealing simulations were performed using the FA / QT-GI / GA method, with an immersion temperature of 840°C and a quenching temperature ranging from 250 to 350°C. As shown in Table C2, this comparative steel R exhibits the following tensile properties: UTS of 1140 to 1285 MPa, significantly lower than the UTS of 1480 to 1750 MPa described in Examples 1 to 11. The lower tensile properties are attributed to a suboptimal composition, such as the lack of vanadium carbide precipitation strengthening.

[0102] Table C2

[0103] Properties of steel R and FA / QT-GI / GA method

[0104]

[0105]

[0106] It should be understood that any or more of the teachings, expressions, embodiments, examples, etc., set forth herein can be combined with any or more of the other teachings, expressions, embodiments, examples, etc., set forth herein. Therefore, the teachings, expressions, embodiments, examples, etc., set forth below should not be viewed in isolation from each other. In view of the teachings herein, those skilled in the art will readily understand various suitable ways in which the teachings herein can be combined. The modifications and variations are intended to be included within the claims.

[0107] Various embodiments of the invention have been shown and described. Further modifications to the methods and systems described herein can be made by those skilled in the art through appropriate modifications without departing from the scope of the invention. Several potential modifications have been mentioned, and others will be apparent to those skilled in the art. For example, the examples, embodiments, geometries, materials, dimensions, ratios, steps, and the like discussed above are illustrative and not essential. Therefore, the scope of the invention should be considered within the following claims and is to be understood as not being limited to the details of the structures and operations shown and described in the specification and drawings.

Claims

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

2. The steel according to claim 1, wherein the steel is formed as a plate.

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

4. The steel according to any one of claims 1 to 3, wherein the ultimate tensile strength is greater than or equal to about 1480 MPa and less than or equal to about 1750 MPa.

5. The steel according to any one of claims 1 to 4, wherein the yield strength is greater than or equal to about 1050 MPa and less than or equal to about 1050 MPa.

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

7. The steel according to any one of claims 1 to 6, wherein the steel microstructure comprises, based on area percentage, about 10 to 25% ferrite, about 75 to 90% martensite, and about 3 to 10% retained austenite.

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

9. The steel according to claim 7 or 8, wherein the martensite is preferably 75 to 80%.

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

11. The steel according to any one of claims 1 to 6, wherein the steel microstructure comprises, based on area percentage, about 20 to 65% tempered martensite, about 35 to 80% newly prepared martensite, and about 3 to 10% retained austenite.

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

13. The steel according to claim 11 or 12, wherein the newly prepared martensite is preferably 50 to 65%.

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

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

16. The steel according to any one of claims 1 to 15, wherein the steel is configured for galvanizing and galvanizing annealing processes.

17. The steel according to any one of claims 1 to 16, having 0.0001-0.003% B.

18. A method for manufacturing steel, comprising: (a) Melting an ingot with the composition of 0.15 to 0.26% C, 2.10 to 3.60% Mn, 0.05 to 0.85% Si, 0.001 to 0.85% Al, 0.01 to 0.90% Cr, 0.01 to 0.50% Mo, 0.01 to 0.10% Ti, 0.01 to 0.04% Nb, 0.01 to 0.30% V, 0.0001 to 0.005% B, less than 0.01% N, less than 0.01% S, less than 0.05% P, with the remainder being Fe and impurities; (b) The ingot is hot rolled into a hot strip with a gauge length of about 3.0 to 4.0 mm, wherein the hot rolling is carried out at about 1260°C, the final temperature is about 850 to 900°C and the coiling temperature is about 600°C; (c) Anneal the tropical material; and (d) The annealed hot strip is cold rolled to reduce the gauge length, thereby forming a sheet with a gauge length of about 1.2 to 2.0 mm.

19. The method of claim 18, wherein the steel is fully annealed in a temperature range of about 840 to 860°C and above the Ac3 temperature, then quenched in a temperature range of about 250 to 375°C, then galvanized and galvanized annealed, and then tempered and rolled.

20. The method of claim 18, wherein the steel is subjected to critical intercalation annealing at about 750 to 830°C and in a temperature range between Ac1 and Ac3, then cooled to below the galvanizing temperature, then galvanized and galvanized annealed, and then tempered and rolled.

21. The method of claim 18, wherein the steel is subjected to critical intercalation annealing in a temperature range of about 750 to 830°C and between Ac1 and Ac3 temperatures, then quenched to a temperature below the galvanizing temperature, then galvanized and galvanized annealed, and then tempered and rolled.

22. The method of claim 18, wherein it has 0.0001 to 0.003% B.

23. The method of claim 18, wherein cold rolling the annealed hot strip to reduce the gauge length enables the formation of a sheet with a gauge length of about 1.2 to 1.6 mm.

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