A low-yield ratio 420MPa low-alloy high-strength steel and preparation method thereof

Abstract

The present application relates to a low-yield ratio 420MPa low-alloy high-strength steel and a preparation method, belonging to the technical field of steel manufacturing. In terms of mass fraction, the chemical composition of the low-alloy high-strength steel includes: C: 0.07% to 0.12%, Si: 0.10% to 0.40%, Mn: 1.60% to 1.80%, P: ≤0.015%, S: ≤0.010%, Alt: 0.020% to 0.060%, Nb: 0.03% to 0.05%, Ti: 0.01% to 0.03%, N≤0.005%, and matrix element Fe. The composition design gives full play to the solid solution strengthening of C and Mn and the fine grain strengthening and precipitation strengthening of Nb and Ti microalloys. At the same time, the continuous full annealing process does not require slow cooling treatment, which improves production efficiency and obtains low-cost, low-yield ratio 420MPa low-alloy high-strength (yield ratio ≤0.8) steel with both strength and excellent plasticity. This type of steel can withstand greater plastic deformation while meeting higher strength requirements without plastic instability, achieving the needs of lightweight automotive industry and excellent stamping integral forming of structural parts, and has broad application prospects.

Description

Technical Field

[0001] The present application relates to the technical field of steel manufacturing, and in particular to a low-yield ratio 420MPa low-alloy high-strength steel and a preparation method thereof. Background Art

[0002] Low alloy high strength steel is a type of steel that increases strength by adding trace alloying elements such as Nb and Ti. It is widely used in automotive structural parts due to its high yield strength and yield strength ratio, good plasticity, toughness, weldability, hot and cold processing and excellent local formability. However, with the demand for energy conservation, emission reduction, lightweight vehicles and cost reduction in the automotive industry, the automotive industry is integrating parts and using one material for multiple purposes, which places higher requirements on the formability of materials.

[0003] Although the low-alloy high-strength 420LA has a high strength-to-yield ratio, its elongation after fracture is low, which is not conducive to overall stamping forming, and cracking sometimes occurs during the stamping and drawing process. Although the same-level 590DP has good plasticity, its yield strength is low, which is not conducive to automobile lightweight control.

[0004] At present, most of the literature on the market is about the development of 420LA high yield ratio materials. China's invention patent 201611138928.1 "A cold-rolled steel plate with a yield strength of 420MPa and its preparation method" discloses a method for preparing a cold-rolled steel plate with a yield strength of 420MPa, which is characterized by a cold-rolled plate with a yield strength of 435-450MPa, a tensile strength of 518-548MPa, an elongation A80 of 25.0%-26.3%, and a yield ratio of >0.8. The lower the yield ratio of the steel plate, the safer and more stable it is during processing, forming, and use. However, if the yield ratio is too low, the utilization rate of the steel is low, and if the yield ratio is too high, it is not conducive to processing, forming, and use. Therefore, high-strength and low-yield ratio steel plates with a suitable yield ratio range of 0.6-0.8 are more suitable for market development needs. Chinese invention patent CN117920755A "A production method for reducing the yield ratio of high-strength steel plates" discloses a method for reducing the yield ratio of high-strength steel plates, which is characterized by: accurately controlling the steel rolling, curling and other process systems to obtain high-strength and low-yield ratio automotive hot-formed steel plates. The produced steel plates have excellent mechanical properties and the yield ratio is ≤0.8. This invention is a method for manufacturing hot-rolled products. Summary of the invention

[0005] The present application provides a low-yield ratio 420MPa low-alloy high-strength steel and a preparation method thereof to solve the following technical problem: how to reduce the yield ratio of low-alloy high-strength steel.

[0006] In the first aspect, an embodiment of the present application provides a low-yield ratio 420 MPa low-alloy high-strength steel. The chemical composition of the low-alloy high-strength steel includes, by mass fraction: C: 0.07%~0.12%, Si: 0.10%~0.40%, Mn: 1.60%~1.80%, P: ≤0.015%, S: ≤0.010%, Alt: 0.020%~0.060%, Nb: 0.03%~0.05%, Ti: 0.01%~0.03%, N≤0.005%, and matrix element Fe.

[0007] Optionally, the thickness of the low-alloy high-strength steel is 0.5 mm to 3.0 mm.

[0008] Optionally, the low-alloy high-strength steel satisfies at least one of the following properties: yield strength of 420MPa to 560MPa, tensile strength ≥590MPa, elongation after fracture ≥16%, and yield strength ratio ≤0.8.

[0009] In a second aspect, an embodiment of the present application provides a method for preparing the low alloy high strength steel described in the first aspect, the method comprising:

[0010] Obtaining a slab having the chemical composition;

[0011] The slab is sequentially heated, rolled, cooled and coiled to obtain a hot-rolled coil;

[0012] Cold rolling the hot rolled coil to obtain a chilled coil;

[0013] The chilled coil is subjected to continuous full annealing and hot-dip galvanizing to obtain a cold-rolled finished coil;

[0014] The cold-rolled finished coil is flattened to obtain low-alloy high-strength steel.

[0015] Optionally, the heating temperature is 1190° C. to 1270° C., and the heating insulation time is ≥160 min.

[0016] Optionally, the final rolling temperature is 850°C to 900°C.

[0017] Optionally, the cooling adopts a front-stage centralized water cooling mode, and the coiling temperature is 500°C to 600°C.

[0018] Optionally, the cold rolling reduction rate is 50% to 75%.

[0019] Optionally, the continuous full annealing includes: heating, soaking and rapid cooling; the soaking temperature is 700°C to 800°C, and the soaking time is 50s to 80s; the cooling rate of the rapid cooling is 6°C / s to 15°C / s, and the rapid cooling temperature is 400°C to 480°C.

[0020] Optionally, the flattening elongation of the flattening treatment is 1.0% to 1.8%.

[0021] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0022] An embodiment of the present application provides a low-yield ratio 420 MPa low-alloy high-strength steel. The chemical composition of the low-alloy high-strength steel includes, by mass fraction: C: 0.07% to 0.12%, Si: 0.10% to 0.40%, Mn: 1.60% to 1.80%, P: ≤0.015%, S: ≤0.010%, Alt: 0.020% to 0.060%, Nb: 0.03% to 0.05%, Ti: 0.01% to 0.03%, N≤0.005%, and matrix element Fe. The micro-alloying composition design of C, Mn, Nb and Ti can give full play to the solid solution strengthening effect of C and Mn to improve the strength of the steel; the fine grain strengthening and precipitation strengthening effects of Nb and Ti micro-alloys can be brought into play to improve the strength and toughness of the steel, and a low-yield ratio 420MPa low-alloy high-strength steel with both high strength and excellent plastic deformation ability is obtained. This steel can withstand greater plastic deformation without plastic instability while meeting higher strength requirements, achieving stable and excellent stamping overall forming requirements, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0024] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0025] Figure 1 A schematic flow chart of a method for preparing a low-yield ratio 420 MPa low-alloy high-strength steel provided in an embodiment of the present application. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0027] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range; in addition, whenever a numerical range is indicated in this document, it is meant to include any cited numbers (fractions or integers) within the indicated range.

[0028] In this document, the terms including "including" and "including" mean "including but not limited to". Relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. "And / or" describes the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "plurality" means two or more; "at least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items; for example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts" such as parts by weight and parts by mass indicate the proportional relationship between the components. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, if the weight ratio of substance A, substance B and substance C is 1:2:3, then substance A, substance B and substance C should correspond to the proportional numbers in the proportional formula in the order of description, that is, the weight of substance A: the weight of substance B: the weight of substance C = 1:2:3.

[0029] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0030] In the first aspect, an embodiment of the present application provides a low-yield ratio 420 MPa low-alloy high-strength steel, wherein the chemical composition of the low-alloy high-strength steel includes, by mass fraction: C: 0.07% to 0.12%, Si: 0.10% to 0.40%, Mn: 1.60% to 1.80%, P: ≤0.015%, S: ≤0.010%, Alt: 0.020% to 0.060%, Nb: 0.03% to 0.05%, Ti: 0.01% to 0.03%, N≤0.005%, and matrix element Fe; wherein Alt represents all aluminum.

[0031] C is the most important solid solution strengthening element. It also forms carbonitrides with microalloy Nb and Ti during heat treatment, which plays a role in fine grain strengthening and precipitation strengthening. However, a C mass fraction higher than 0.12% will affect welding performance and forming quality. Therefore, the C mass fraction is limited to 0.07% to 0.12% in the embodiments of the present application. For example, the C mass fraction can be 0.07%, 0.08%, 0.09%, 0.10%, 0.11%, 0.12%, etc.

[0032] Si is also one of the main solid solution strengthening elements, but it does not form carbides in steel, but dissolves in ferrite or austenite in the form of solid solution, which can reduce the austenite phase area. Increasing Si can improve the hardness, strength and high temperature fatigue strength of steel, but it will deteriorate the welding performance of steel. Therefore, in the embodiment of the present application, the mass fraction of Si is limited to 0.1% to 0.4%. For example, the mass fraction of Si can be 0.10%, 0.20%, 0.30%, 0.40%, etc.

[0033] Mn is also a solid solution strengthening element, which can expand the austenite area and form MnS with sulfur to prevent hot brittleness caused by sulfur, thereby improving the hot working performance of steel. However, Mn is easy to cause structural segregation, and a mass fraction higher than 1.80% is easy to cause forming cracking, deteriorate the comprehensive performance of steel, and also affect welding performance. Therefore, the mass fraction of Mn is limited to 1.6% to 1.8% in the embodiment of the present application. For example, the mass fraction of Mn can be 1.60%, 1.65%, 1.70%, 1.75%, 1.80%, etc.

[0034] P also plays a role in solid solution strengthening, but while improving strength, P causes a significant decrease in plasticity and impact toughness, deteriorating the mechanical properties of the material. Therefore, in the embodiments of the present application, the mass fraction of P is limited to ≤ 0.015%. For example, the mass fraction of P can be 0.007%, 0.008%, 0.009%, 0.010%, 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, etc.

[0035] S is derived from molten iron and steelmaking raw materials and is a harmful element. FeS formed in molten steel is prone to premature melting, resulting in cracking of the workpiece, while reducing the ductility and toughness of the steel. Therefore, the mass fraction of S is limited to ≤0.010% in the embodiments of the present application. For example, the mass fraction of S can be 0.002%, 0.004%, 0.006%, 0.008%, 0.010%, etc.

[0036] Al is mainly used for deoxidation and alloying. When alloying elements are added to steel, they can refine the grains and fix nitrogen in the steel, thereby inhibiting the aging of low-carbon steel. In the embodiment of the present application, Alt is limited to 0.020% to 0.060%. For example, the mass fraction of Alt can be 0.020%, 0.030%, 0.040%, 0.050%, 0.060%, etc. Alt (Altotal) refers to the total aluminum content in steel, that is, all aluminum elements, including aluminum present in steel in various forms.

[0037] Ti is one of the commonly used microalloying elements in the metallurgical industry. Ti can improve plasticity and toughness in low alloy steel. Since Ti fixes nitrogen and sulfur and forms a titanium carbide nano-precipitation phase, it plays a role in grain refinement and precipitation strengthening. Precipitation to form carbides can significantly improve the plasticity and impact toughness of steel. Alloy steel containing Ti has good mechanical properties and process properties, but Ti is easily oxidized and has a complex morphology. The mass fraction is higher than 0.03%, which affects the performance stability. Therefore, the mass fraction of Ti in the embodiment of the present application is limited to 0.01% to 0.03%. Exemplarily, the mass fraction of Ti can be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, etc.

[0038] Nb is also a microalloying element. Nb mainly exists in the form of carbides. It refines the grains, improves the impact toughness of steel and reduces its brittle transition temperature. However, when the mass fraction of Nb is higher than 0.05%, the cost is increased. Therefore, the mass fraction of Nb in the present application is limited to 0.03% to 0.05%. For example, the mass fraction of Nb can be 0.03%, 0.035%, 0.04%, 0.045%, 0.05%, etc.

[0039] N can be dissolved in iron to form an interstitial solid solution, but while the strength and hardness are improved, the toughness of the steel decreases and the notch sensitivity increases. In the embodiment of the present application, the mass fraction of N is ≤0.005%.

[0040] Fe is a matrix element. The specific content / content range of Fe can be obtained by the upper and lower limit formula of the component, that is:

[0041] The sum of the percentages of the contents of each component in a composition should be equal to 100%, and the content ranges of several components should meet the following conditions: the upper limit value of a component + the lower limit value of other components ≤ 100; the lower limit value of a component + the upper limit value of other components ≥ 100.

[0042] In some embodiments, the low alloy high strength steel has a thickness of 0.5 mm to 3.0 mm.

[0043] In some embodiments, the low alloy high strength steel satisfies at least one of the following properties: yield strength of 420MPa to 560MPa, tensile strength ≥590MPa, elongation after fracture ≥16%, and yield strength ratio ≤0.8.

[0044] The yield strength ratio refers to the ratio of the yield strength of a material to its tensile strength. It is a dimensionless number that reflects the material's ability to transition from elastic deformation to plastic deformation under the action of external forces. The yield strength ratio can be used as a coefficient to measure the strength reserve of metal materials such as steel. The smaller the yield strength ratio, the better the plasticity of the material and the easier it is to deform; the larger the yield strength ratio, the stronger the material's ability to resist deformation and the less likely it is to undergo plastic deformation.

[0045] In summary, the low-yield ratio 420 MPa low-alloy high-strength steel provided in the embodiments of the present application has excellent mechanical properties and chemical composition, and is suitable for various application scenarios requiring high strength, good toughness and corrosion resistance.

[0046] Figure 1 A schematic flow chart of a method for preparing a low-yield ratio 420 MPa low-alloy high-strength steel provided in an embodiment of the present application.

[0047] See also Figure 1 In a second aspect, an embodiment of the present application provides a method for preparing a low-yield ratio 420 MPa low-alloy high-strength steel as described in the first aspect, the method comprising:

[0048] S1, obtaining a slab having the chemical composition;

[0049] In the embodiment of the present application, the molten iron is subjected to KR desulfurization, converter smelting, and then refined and continuously cast to obtain a slab with the chemical composition, which ensures the performance of the final product. KR desulfurization, namely KR (Kambara Reactor) desulfurization, is an important molten iron pretreatment desulfurization process.

[0050] S2, sequentially heating, rolling, cooling and coiling the slab to obtain a hot-rolled coil;

[0051] In some embodiments, the heating temperature is 1190° C. to 1270° C., and the heating insulation time is ≥160 min.

[0052] The positive effect of limiting the heating temperature to 1190°C to 1270°C: Within this temperature range, microalloying elements such as Nb (niobium) and Ti (titanium) can be fully dissolved in the austenite matrix. These elements play a role in refining grains, improving strength and toughness in steel. By heating and heat preservation, it can be ensured that the solubility of these elements in austenite is maximized, thereby achieving the best strengthening effect in the subsequent cooling and rolling processes. Exemplarily, the heating temperature can be 1190°C, 1210°C, 1230°C, 1250°C, 1270°C, etc.

[0053] The length of the holding time directly affects the dissolution degree and distribution uniformity of microalloying elements (such as Nb, Ti, etc.) in the steel. Holding time ≥ 160 min can ensure that these elements are fully dissolved in the austenite matrix and reach a uniform distribution state in the entire steel billet, which is crucial for the precipitation strengthening effect of microalloying elements in subsequent process steps. Exemplarily, the holding time for heating can be 160 min, 170 min, 180 min, 190 min, 200 min, etc.

[0054] In some embodiments, the final rolling temperature of the rolling is 850°C to 900°C.

[0055] Final rolling within this temperature range can ensure that austenite recrystallizes during the rolling process. Austenite recrystallization helps to refine the grains and improve the mechanical properties and formability of the steel. At the same time, the recrystallization process can also eliminate or reduce the internal stress and work hardening generated during the rolling process. The choice of final rolling temperature also affects the subsequent phase transformation process. Final rolling within the temperature range of 850°C to 900°C can provide suitable conditions for subsequent cooling and phase transformation processes. By controlling the final rolling temperature, the microstructure of the steel can be adjusted, thereby affecting its mechanical properties and corrosion resistance. Exemplarily, the final rolling temperature can be 850°C, 860°C, 870°C, 880°C, 890°C, 900°C, etc.

[0056] In some embodiments, the cooling adopts a front-stage centralized water cooling mode.

[0057] The front-end concentrated water cooling mode refers to the process of controlling the opening sequence and number of the manifold valves in the laminar cooling zone during the cooling of hot-rolled steel, so as to concentrate the rapid cooling area in the front of the laminar cooling zone to achieve uniform cooling and performance optimization of the steel. The main features of this mode are fast cooling speed, high cooling efficiency, and targeted cooling control for different parts of the steel. The front-end concentrated water cooling mode helps to refine the grains of steel and improve the strength and toughness of steel. Through rapid cooling, the transformation of austenite to pearlite can be suppressed and the formation of ferrite can be promoted, thereby obtaining a more uniform and fine structure. The front-end concentrated water cooling mode also helps to reduce defects such as cracks and inclusions generated in the hot rolling process of steel. Rapid cooling can suppress the formation and expansion of these defects, thereby improving the quality and reliability of steel.

[0058] In some embodiments, the coiling temperature is 500°C to 600°C.

[0059] Within this temperature range, it can be ensured that the C and N compounds of Nb and Ti elements can be fully and evenly precipitated to achieve the purpose of improving the strength of the steel strip. For example, the coiling temperature can be 500°C, 520°C, 540°C, 560°C, 580°C, 600°C, etc.

[0060] S3, cold rolling the hot rolled coil to obtain a chilled coil;

[0061] In some embodiments, the cold rolling reduction ratio is 50% to 75%.

[0062] In the embodiment of the present application, the hot rolled coil is pickled and then cold rolled, and the reduction rate of the cold rolling process is 50% to 75%, the main purpose of which is to provide sufficient recrystallization driving force for subsequent annealing to ensure that the cold rolling annealing quickly realizes structural recrystallization and homogenization, shortens the annealing time, and avoids the use of high annealing temperature, thereby achieving the effect of reducing costs. Exemplarily, the reduction rate of cold rolling can be 50%, 55%, 60%, 65%, 70%, 75%, etc.

[0063] S4, performing continuous full annealing and hot-dip galvanizing on the chilled coil to obtain a cold-rolled finished coil;

[0064] In some embodiments, the continuous full annealing includes: heating, soaking and rapid cooling; the soaking temperature is 700°C to 800°C, and the soaking time is 50s to 80s; the cooling rate of the rapid cooling is 6°C / s to 15°C / s, and the rapid cooling temperature is 400°C, 420°C, 440°C, 460°C, 480°C, etc.

[0065] The continuous full annealing process limits the soaking temperature to 700℃~800℃ and the soaking time to 50s~80s, so as to fully eliminate the internal stress and residual stress in the cold hardened coil, reduce the hardness of the steel coil, improve the plate shape, effectively eliminate the fibrous structure in the organization of the cold hardened coil, refine the grains in the steel coil, make the organization more uniform, thereby improving the strength and toughness of the steel coil and preventing stamping cracking. Exemplarily, the soaking temperature can be 700℃, 720℃, 740℃, 760℃, 780℃, 800℃, etc.

[0066] In the embodiment of the present application, after soaking, the steel strip is rapidly cooled to 440°C to 480°C at a cooling rate of 6°C / s to 15°C / s for galvanizing, and the rapid cooling temperature is appropriately controlled to ensure the quality of the surface coating of the steel strip. For example, the rapid cooling temperature can be 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, etc.

[0067] S5. Flatten the cold-rolled finished coil to obtain low-alloy high-strength steel.

[0068] In some embodiments, the flattening elongation of the flattening treatment is 1.0% to 1.8%.

[0069] The flattening adopts the elongation mode control, which can effectively control the dimensional accuracy, eliminate the yield platform and improve the forming performance. If the flattening elongation is less than 1.0%, it is impossible to improve the plate shape and eliminate the yield platform. The stamping forming is prone to the risk of orange peel. If the rolling force is too large, it will deteriorate the plate shape and increase the equipment load. Therefore, the flattening elongation is limited to 1.0% to 1.8%. Exemplarily, the flattening elongation can be 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, etc.

[0070] The product prepared by the preparation method of the low-yield ratio 420MPa low-alloy high-strength steel is the above-mentioned low-yield ratio 420MPa low-alloy high-strength steel. Since the preparation method of the low-yield ratio 420MPa low-alloy high-strength steel adopts part or all of the technical solutions of the low-yield ratio 420MPa low-alloy high-strength steel embodiment, it at least has all the beneficial effects brought by the technical solutions of the low-yield ratio 420MPa low-alloy high-strength steel embodiment, which will not be repeated here one by one.

[0071] The present application is further described below in conjunction with specific embodiments. The experimental methods in the following embodiments that do not specify specific conditions are usually measured in accordance with national standards / industry standards; if there are no corresponding national standards / industry standards, they are measured in accordance with common international standards, conventional conditions, or conditions recommended by the manufacturer.

[0072] The molten iron is subjected to KR desulfurization, converter smelting, refining and continuous casting to obtain a slab with high cleanliness. The chemical composition (wt%) of the slab is shown in Table 1.

[0073] Table 1 Chemical composition of slab (wt%), the rest is Fe and unavoidable impurities

[0074] Group C％ Si% Mn% P％ S％ Al％ Nb% Ti% N％ Example 1 0.12 0.25 1.74 0.015 0.007 0.044 0.035 0.017 0.0050 Example 2 0.10 0.36 1.63 0.007 0.001 0.024 0.041 0.011 0.0018 Example 3 0.07 0.21 1.68 0.012 0.004 0.043 0.045 0.012 0.0034 Example 4 0.08 0.22 1.71 0.012 0.005 0.037 0.049 0.026 0.0023 Example 5 0.10 0.19 1.70 0.011 0.001 0.032 0.036 0.024 0.0034 Example 6 0.09 0.18 1.71 0.009 0.007 0.057 0.044 0.019 0.0029 Example 7 0.11 0.38 1.67 0.014 0.003 0.034 0.034 0.030 0.0041 Example 8 0.10 0.29 1.67 0.011 0.005 0.048 0.044 0.029 0.0040 Example 9 0.09 0.19 1.79 0.011 0.005 0.039 0.045 0.023 0.0034 Example 10 0.12 0.18 1.69 0.013 0.005 0.054 0.046 0.022 0.0046 Comparative Example 1 0.11 0.17 1.50 0.012 0.003 0.027 0.025 0.022 0.0040 Comparative Example 2 0.13 0.18 1.85 0.011 0.003 0.037 0.054 0.017 0.0040 Comparative Example 3 0.010 0.20 1.70 0.011 0.005 0.038 0.035 0.023 0.0034

[0075] Based on the chemical composition of the slabs of the embodiment and the comparative example, this embodiment also provides a method for preparing a low-yield ratio 420 MPa low-alloy high-strength steel, comprising the following steps:

[0076] Obtaining a slab having the chemical composition;

[0077] The slab is sequentially heated, rolled, cooled and coiled to obtain a hot-rolled coil;

[0078] Cold rolling the hot rolled coil to obtain a chilled coil;

[0079] The chilled coil is subjected to continuous full annealing and hot-dip galvanizing to obtain a cold-rolled finished coil;

[0080] The cold-rolled finished coil is flattened to obtain low-alloy high-strength steel. Please refer to Table 2 and Table 3 for the preparation process parameters.

[0081] Table 2 Preparation process parameters of examples and comparative examples

[0082]

[0083]

[0084] Table 3 Preparation process parameters of examples and comparative examples

[0085]

[0086] Table 4 Performance list of embodiments and comparative examples

[0087] Group Yield strength / MPa Tensile strength / MPa A80 / ％ Yield-to-strength ratio Example 1 448 642 24 0.70 Example 2 456 648 24 0.70 Example 3 448 632 25 0.71 Example 4 454 626 25 0.72 Example 5 456 627 26 0.73 Example 6 454 625 26 0.73 Example 7 462 628 25 0.74 Example 8 475 640 28 0.74 Example 9 524 664 25 0.79 Example 10 452 623 27 0.73 Comparative Example 1 415 507 28 0.82 Comparative Example 2 576 748 17 0.77 Comparative Example 3 411 577 29 0.71

[0088] The mechanical properties of the embodiments and comparative examples are shown in Table 4. As can be seen from Tables 1 to 4, the 420MPa low alloy high strength steel prepared by the method provided by the embodiment of the present invention has a yield strength of 420MPa to 560MPa, a tensile strength of ≥590MPa, an elongation after fracture of ≥16%, and a yield strength ratio of ≤0.8, which has both high strength and excellent plastic deformation ability, can withstand greater plastic deformation without plastic instability while meeting higher strength, and achieves stable and excellent stamping integral forming requirements.

[0089] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0090] The continuous full annealing process of the embodiment of the present invention does not require slow cooling treatment, shortens the production process, reduces production costs, and meets the needs of the automotive industry for lightweight and excellent stamping integral forming of automotive structural parts.

[0091] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. It will be apparent to those skilled in the art that various modifications to these embodiments are possible, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown in the present application, but will conform to the widest range consistent with the principles and novel features applied for by the present application.

Claims

1. A low-yield ratio 420MPa low-alloy high-strength steel, the chemical composition of which, by mass fraction, comprises: C： 0.07%~0.12%, Si: 0.10%~0.40%, Mn: 1.60%~1.80%, P: ≤0.015%, S: ≤0.010%, Alt: 0.020%~0.060%, Nb: 0.03%~0.05%, Ti: 0.01%~0.03%, N≤0.005%, and matrix element Fe.

2. The low alloy high strength steel according to claim 1, characterized in that: The thickness of the low alloy high strength steel is 0.5 mm to 3.0 mm.

3. The low alloy high strength steel according to claim 1, characterized in that: The low alloy high strength steel meets at least one of the following properties: yield strength of 420MPa to 560MPa, tensile strength ≥590MPa, elongation after fracture ≥16%, and yield strength ratio ≤0.

8.

4. A method for preparing low alloy high strength steel according to any one of claims 1 to 3, characterized in that: The method comprises: Obtaining a slab having the chemical composition; The slab is sequentially heated, rolled, cooled and coiled to obtain a hot-rolled coil; Cold rolling the hot rolled coil to obtain a chilled coil; The chilled coil is subjected to continuous full annealing and hot-dip galvanizing to obtain a cold-rolled finished coil; The cold-rolled finished coil is flattened to obtain low-alloy high-strength steel.

5. The method according to claim 4, characterized in that The heating temperature is 1190° C. to 1270° C., and the heating insulation time is ≥160 min.

6. The method according to claim 4, characterized in that The final rolling temperature of the rolling is 850°C to 900°C.

7. The method according to claim 4, characterized in that The cooling adopts a front-stage concentrated water cooling mode, and the coiling temperature is 500°C to 600°C.

8. The method according to claim 4, characterized in that The cold rolling reduction ratio is 50% to 75%.

9. The method according to claim 4, characterized in that The continuous full annealing includes: heating, soaking and rapid cooling; and / or, The soaking temperature is 700°C to 800°C, and the soaking time is 50s to 80s; The cooling rate of the rapid cooling is 6°C / s to 15°C / s, and the temperature of the rapid cooling is 400°C to 480°C.

10. The method according to claim 4, characterized in that The flattening elongation of the flattening treatment is 1.0% to 1.8%.

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