High-ductility 590MPa dual-phase steel and preparation method thereof

Abstract

The invention provides high-ductility 590 MPa dual-phase steel and a preparation method thereof, and belongs to the field of steel preparation. The dual-phase steel comprises the following chemical components: 0.07%-0.09% of C, 0.3%-0.5% of Si, 1.5%-1.8% of Mn, 0.03%-0.07% of Alt, less than or equal to 0.002% of P, less than or equal to 0.007% of S and Fe. According to the production method of the 590MPa-grade low-cost high-efficiency dual-phase steel for the automobile, which is obtained by adopting the component design with reasonable proportion, adopting higher turnover efficiency and matching with reasonable hot rolling, controlled rolling and annealing processes, the elongation is higher, the yield ratio is lower, and the mechanical property meets the technical requirement and the use requirement of a user; and meanwhile, the alloy disclosed by the invention is low in cost, simple in preparation method and suitable for industrial production.

Description

Technical Field

[0001] The present application relates to the technical field of steel preparation, and in particular to a high-ductility 590 MPa dual-phase steel and a preparation method thereof. Background Art

[0002] 590MPa duplex steel is composed of both ferrite and martensite. This unique microstructure allows duplex steel to maintain high strength while also offering excellent formability and weldability. Consequently, duplex steel has found widespread application in the automotive industry. It is primarily used in the manufacture of components such as vehicle body structures, chassis, and suspension systems, contributing to improved vehicle lightweighting, safety, and durability.

[0003] With the increasing development of automobile models and the diversification of part styles, the demand for material forming is increasing. Previous ductile materials cannot meet customers' stamping forming performance requirements. The cold-rolled product DP590 produced within the performance range of national and enterprise standards can no longer meet customers' diverse parts production needs. It suffers from problems such as stamping cracking and springback, thus requiring low-yield, high-elongation products. In recent years, automakers have proposed 590MPa dual-phase steel with an elongation requirement of >27% to meet application requirements. According to previous production processes, the performance qualification rate is less than 50%, so it is necessary to develop low-yield, high-ductility products to meet application requirements. Summary of the Invention

[0004] The present application provides a high-ductility 590 MPa dual-phase steel and a preparation method thereof to solve the following technical problem: how to improve the ductility while reducing the yield strength ratio of the 590 MPa dual-phase steel.

[0005] In a first aspect, an embodiment of the present application provides a high-ductility 590 MPa dual-phase steel. The chemical composition of the dual-phase steel includes, by mass fraction: C: 0.07% to 0.09%, Si: 0.3% to 0.5%, Mn: 1.5% to 1.8%, Alt: 0.03% to 0.07%, P≤0.002%, S≤0.007%, Fe.

[0006] Optionally, the metallographic structure of the dual-phase steel includes, by volume fraction, ferrite: 85% to 95%, and martensite: 5% to 15%.

[0007] Optionally, the dual-phase steel meets the following properties: yield strength of 340 MPa to 390 MPa, tensile strength ≥ 590 MPa, and elongation A80 ≥ 27%.

[0008] Optionally, the thickness of the duplex steel is ≤2.6 mm.

[0009] In a second aspect, the present application provides a method for preparing the dual-phase steel according to any one embodiment of the first aspect, the method comprising:

[0010] Obtaining a casting billet having the chemical composition;

[0011] The ingot is directly charged into a furnace at a set temperature for heating in the furnace, and then rolled to obtain a hot-rolled steel strip with a set thickness;

[0012] The hot-rolled steel strip is cooled and coiled in a front-end intensive cooling mode to obtain a hot-rolled coil;

[0013] Pickling and cold rolling the hot rolled coil to obtain a chilled steel strip;

[0014] The chilled steel coil is continuously annealed and skin-passed to obtain the dual-phase steel.

[0015] Optionally, the set temperature is ≥800°C, the holding time of the heating in the furnace is 160min to 180min, and the outlet temperature of the heating in the furnace is 1180°C to 1220°C.

[0016] Optionally, the finishing temperature of the rolling is 870° C. to 910° C., and the set thickness is 2.5 mm to 5.5 mm.

[0017] Optionally, the cooling speed is 25°C / s to 35°C / s, and the coiling temperature is 530°C to 570°C.

[0018] Optionally, the step of continuously annealing and skin-passing the chilled steel strip to obtain the dual-phase steel comprises:

[0019] Heating the chilled steel strip; the heating temperature is 780° C. to 820° C.;

[0020] The heated chilled steel strip is subjected to uniform heating at a temperature of 780° C. to 820° C.

[0021] Slowly cooling the chilled steel strip after soaking; the slow cooling temperature is 580° C. to 620° C.;

[0022] The chilled steel strip after slow cooling is rapidly cooled; the rapid cooling temperature is 300° C. to 330° C.;

[0023] The chilled steel strip after rapid cooling is subjected to an aging treatment; the temperature of the aging treatment is 270° C. to 310° C.;

[0024] The over-aging treated chilled steel coil is skin-passed to obtain the dual-phase steel.

[0025] Optionally, the belt speed of the continuous annealing is 50 m / min to 200 m / min, and the flattening elongation is 0.2% to 0.5%.

[0026] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0027] The present invention provides a high-ductility 590 MPa dual-phase steel. The chemical composition of the dual-phase steel is rationally designed. First, carbon and manganese synergistically stabilize the austenite phase, while silicon and alt inhibit carbide precipitation, promoting carbon enrichment in the austenite. During cooling, stable supercooled austenite is formed, and during the phase transformation, a certain proportion of stable martensite is formed. This improves elongation while meeting strength requirements. Secondly, the carbon content is controlled to an upper limit of ≤ 0.09% to limit excessive martensite formation and avoid excessive strength leading to a decrease in ductility. Si increases the dislocation density of ferrite, reducing yield strength while maintaining tensile strength, directly reducing the yield strength ratio. Simultaneously, the high dislocation density promotes uniform plastic deformation and improves elongation. Finally, phosphorus and sulfur are controlled to ultra-low levels to eliminate brittle phases and grain boundary segregation caused by impurities, ensuring that plastic deformation capacity is not impaired by impurities, providing a foundation for high elongation. This reduces the yield strength ratio of the 590 MPa dual-phase steel while improving ductility. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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.

[0029] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0030] Figure 1 A schematic flow chart of a method for preparing duplex steel provided in an embodiment of the present application;

[0031] Figure 2 The metallographic structure diagram of the duplex steel provided in Example 1 of the present application;

[0032] Figure 3 The metallographic structure diagram of the duplex steel provided for the comparative example of this application. DETAILED DESCRIPTION

[0033] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0034] 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 hard limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numbers within the range. For example, the description of a range from 1 to 6 should be considered to have 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 herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0035] In addition, in the description of the specification of this application, the terms "including", "comprising", etc. mean "including but not limited to". In this article, relational terms such as "first" and "second" are merely 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. In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A and B can be singular or plural. In this article, "at least one" refers to one or more, and "plurality" refers to 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 represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple. "Parts" such as parts by weight and parts by mass represent 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 mass ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one-to-one to the proportional numbers in the proportional formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0036] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0037] The present application provides a high-ductility 590MPa dual-phase steel. The chemical composition of the dual-phase steel includes, by mass fraction: C: 0.07% to 0.09%, Si: 0.3% to 0.5%, Mn: 1.5% to 1.8%, Alt: 0.03% to 0.07%, P≤0.002%, S≤0.007%, and Fe.

[0038] It should be noted that Fe is a matrix element, and the specific content / content range of Fe can be obtained by the upper and lower limit formula of the components, that is: the sum of the percentage content of each component in a composition should be equal to 100%, and the content range 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.

[0039] The functions of each element are as follows:

[0040] Carbon stabilizes austenite. If the carbon content is too low, the tensile strength will not meet standard requirements. If the carbon content is too high, the proportion of martensite in the finished product increases, resulting in excessive strength and low elongation. For example, the mass fraction of carbon can be 0.07%, 0.075%, 0.08%, 0.085%, 0.09%, etc.

[0041] Si inhibits carbide precipitation, promotes carbon enrichment in austenite, and facilitates the formation of retained austenite. This can result in a higher dislocation density in ferrite, thereby increasing the work hardening rate and improving elongation. However, if the Si content is too low, insufficient carbon in the austenite results in a low amount of retained austenite, which is detrimental to improving elongation and hole expansion performance. If the Si content is too high, selective oxidation of the strip surface can occur, resulting in color defects after annealing. For example, the mass fraction of Si can be 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, etc.

[0042] Mn stabilizes austenite. If the Mn content is too low, the tensile strength will not meet standard requirements. If the Mn content is too high, the ferrite content in the finished product increases, resulting in excessive strength and low elongation. Furthermore, the higher the Mn content, the more pronounced the banding and the poorer the formability. For example, the mass fraction of Mn can be 1.5%, 1.55%, 1.6%, 1.65%, 1.7%, 1.75%, 1.8%, etc.

[0043] Alt, similar to Si, inhibits carbide precipitation, promotes carbon enrichment in austenite, and facilitates the formation of retained austenite. If the Alt content is too low, the retained austenite ratio decreases, resulting in low elongation and insufficient hole expansion performance. If the Alt content is too high, AltN precipitates form during the casting process, affecting the surface quality of the ingot and causing clustered warping defects. For example, the mass fraction of Alt can be 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, etc.

[0044] P and S elements are impurity elements, and are as low as possible. If the content of P is too high, the strength and hardness of the steel material will increase, the plasticity and toughness will decrease, and the steel material will have cold brittleness, which is not conducive to stamping forming. If the content of S is too high, sulfides will be formed and distributed in the rolling direction in the form of sheets. The sulfides have thermal brittleness, and FeS and molten iron can be infinitely soluble at high temperature. When the solution pool solidifies, FeS and Fe or FeO form a low-melting-point eutectic, which forms a crack source in the welding process after the customer produces automobile parts. For example, the mass fraction of P can be 0.0005%, 0.0008%, 0.001%, 0.0015%, 0.002%, etc. The mass fraction of S can be 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, etc.

[0045] In some embodiments, the metallographic structure of the dual-phase steel includes, in terms of volume fraction: ferrite: 85%-95%, and martensite: 5%-15%.

[0046] Through industrialized full-process production and control of related key process parameters, the proportion of the microstructure in the high-strength steel substrate and the distribution of grain boundaries and dislocation structures can be effectively controlled, and the related physical properties of the material can be realized by improving the microstructure of the material. For example, the volume fraction of ferrite can be 85%, 87%, 90%, 92%, 94%, 95%, etc., and the volume fraction of martensite can be 5%, 7%, 9%, 11%, 13%, 15%, etc.

[0047] In some embodiments, the dual-phase steel satisfies the following properties: yield strength: 340 MPa-390 MPa, tensile strength: ≥590 MPa, and elongation A80: ≥27%.

[0048] By shortening the transfer time from the slab to the hot coil, low-temperature discharge, and short in-furnace time, low production cost and high production efficiency are realized. By optimizing the alloy composition and optimizing the annealing process, low yield strength and high ductility are realized, which meets the requirements of customers for personalized and complex forming parts.

[0049] In some embodiments, the thickness of the dual-phase steel is ≤2.6 mm.

[0050] Figure 1 A flowchart of a preparation method of a dual-phase steel provided in an embodiment of the present application is shown.

[0051] Based on a general inventive concept, as shown in Figure 1 The present application provides a preparation method of a dual-phase steel according to any one of the embodiments of the first aspect, and the method includes:

[0052] S1, obtaining a casting blank having the chemical composition;

[0053] S2. Using a direct loading method, the ingot is charged into a furnace at a set temperature for heating in the furnace, and then rolled to obtain a hot-rolled steel strip with a set thickness;

[0054] In some embodiments, the set temperature is ≥800°C, the holding time of the heating in the furnace is 160 min to 180 min, and the outlet temperature of the heating in the furnace is 1180°C to 1220°C.

[0055] The charging temperature is limited to no less than 800°C to reduce heat loss and improve production efficiency. Below 800°C, aluminum nitride precipitates, which can easily cause transverse cracks in the ingot. Exemplary charging temperatures include 800°C, 820°C, 840°C, and 860°C.

[0056] The furnace discharge temperature for heating is limited to 1180°C to 1220°C. If the discharge temperature is too low, the hot rolling force will be too high, exceeding the mill load. If the discharge temperature is too high, the olivine phase will form on the strip surface, which is difficult to remove and will cause color defects on the surface. For example, the furnace discharge temperature can be 1180°C, 1190°C, 1200°C, 1210°C, 1220°C, etc.

[0057] The holding time for furnace heating of the slab is limited to 160-180 minutes. Too short a holding time in the furnace prevents the interior of the slab from being heated thoroughly, resulting in a mixed crystal structure. Too long a holding time wastes heat energy and reduces rolling efficiency. For example, the holding time for furnace heating can be 160, 165, 170, 175, or 180 minutes.

[0058] In some embodiments, the finishing temperature of the finishing rolling is 870° C. to 910° C., and the set thickness is 2.5 mm to 5.5 mm.

[0059] The finishing temperature for finishing rolling is limited to 870°C to 910°C. If the finishing temperature is too low, the edge temperature drops too quickly, causing the two-phase region to form a mixed grain structure. If the finishing temperature is too high, the rolling speed will be too high, which may lead to the risk of scrapping. For example, the finishing temperature for finishing rolling can be 870°C, 880°C, 890°C, 900°C, 910°C, etc.

[0060] S3, cooling and coiling the hot-rolled steel strip using a front-end intensive cooling mode to obtain a hot-rolled coil;

[0061] In some embodiments, the cooling rate is 25°C / s to 35°C / s, and the coiling temperature is 530°C to 570°C.

[0062] The temperature of the coiling is limited to 530-570℃. If the temperature of the coiling is too low, the strength of the strip is too high, the shape of the coil cannot be guaranteed during the hot rolling and coiling process, the shape of the plate cannot be guaranteed during the pickling and rolling process, and if the temperature of the coiling is too high, the microstructure of the strip is not uniform in the middle and the edge of the plate, and color difference defects are generated. For example, the temperature of the coiling can be 530℃, 540℃, 550℃, 560℃, 570℃, etc.

[0063] S4. Pickling and cold rolling the hot rolled coil to obtain a cold hard coil strip;

[0064] S5. Continuously annealing and finishing the cold hard coil strip to obtain the dual-phase steel.

[0065] In some embodiments, the continuously annealing and finishing the cold hard coil strip to obtain the dual-phase steel comprises:

[0066] heating the cold hard coil strip; the temperature of the heating is 780-820℃;

[0067] soaking the heated cold hard coil strip; the temperature of the soaking is 780-820℃;

[0068] slow cooling the soaked cold hard coil strip; the temperature of the slow cooling is 580-620℃;

[0069] fast cooling the slow cooled cold hard coil strip; the temperature of the fast cooling is 300-330℃;

[0070] overaging the fast cooled cold hard coil strip; the temperature of the overaging is 270-310℃;

[0071] finishing the overaged cold hard coil strip to obtain the dual-phase steel.

[0072] The temperature of the heating and soaking is limited to 780-820℃. If the temperature of the heating and soaking is too low, the strip cannot be fully austenitized, the microstructure of the finished product cannot obtain a certain proportion of martensite, and the standard tensile strength cannot be reached above 590MPa, and if the temperature of the heating is too high, the strip surface is severely overoxidized, and color difference defects are generated. For example, the temperature of the heating and soaking can be 780℃, 790℃, 800℃, 810℃, 820℃, etc.

[0073] The slow cooling temperature is limited to 580℃~620℃. If the slow cooling temperature is lower than 580℃, the finished product will have too much ferrite and too little martensite, and the standard tensile strength of 590MPa or above cannot be achieved. If the slow cooling temperature is higher than 620℃, the ferrite will be too little and the martensite will be too much, resulting in high strength and low elongation, which may cause stamping cracking. For example, the slow cooling temperature can be 580℃, 590℃, 600℃, 610℃, 620℃, etc.

[0074] The temperature during the rapid cooling stage is limited to 300°C to 330°C to control the proportion of martensite. If the rapid cooling temperature is too low, the martensite proportion increases, resulting in excessively high tensile strength. If the rapid cooling temperature is too high, the martensite proportion decreases, and the tensile strength fails to reach the standard tensile strength of 590 MPa or above. For example, the temperature during the rapid cooling stage can be 300°C, 310°C, 320°C, 325°C, 330°C, etc.

[0075] The overaging temperature is limited to 270°C to 310°C. If the overaging temperature is lower than 270°C, the hardness difference between the two phases cannot be reduced. If it is higher than 310°C, the yield strength increases too much, causing springback during stamping. For example, the overaging temperature can be 270°C, 280°C, 290°C, 300°C, 310°C, etc.

[0076] In some embodiments, the belt speed of the continuous annealing is 50 m / min to 200 m / min, and the flattening elongation is 0.2% to 0.5%.

[0077] The strip speed during continuous annealing is limited to 50m / min to 200m / min. If the strip speed is faster than 200m / min, the slow cooling and rapid cooling temperatures cannot meet the required ranges, resulting in excessively high strength and low elongation. If the strip speed is slower than 50m / min, the strip strength is too low to reach the standard tensile strength of 590MPa or above. Exemplary strip speeds during continuous annealing are 50m / min, 70m / min, 90m / min, 120m / min, 150m / min, 180m / min, 200m / min, etc.

[0078] The flattening elongation is limited to 0.2% to 0.5%. If the flattening elongation is lower than 0.2%, the surface roughness will not be fully replicated and the customer's lower roughness requirement will not be met. If the flattening elongation is higher than 0.5%, the yield strength will be too high, causing springback during the stamping process. For example, the flattening elongation can be 0.2%, 0.3%, 0.4%, 0.45%, 0.5%, etc.

[0079] Based on existing processes, this embodiment of the application uses fewer alloys, shortens the transfer time from slab to hot coil, and uses low-temperature furnace discharge and short furnace time, thereby achieving lower production costs. Furthermore, by optimizing the annealing process, lower yield strength and higher elongation are achieved, thus meeting high-quality requirements and creating favorable conditions for continuous and stable production.

[0080] The product produced by the method for preparing dual-phase steel is the aforementioned dual-phase steel. The chemical composition and microstructure of the dual-phase steel produced by the method for preparing dual-phase steel can refer to the aforementioned embodiments. Since the method for preparing dual-phase steel adopts part or all of the technical solutions of the dual-phase steel embodiments, it has at least all the beneficial effects brought about by the technical solutions of the dual-phase steel embodiments, which will not be described in detail here.

[0081] In summary, the high-ductility 590 MPa dual-phase steel and its preparation method provided in the embodiments of the present application have the following advantages:

[0082] (1) Chemical composition optimization: By precisely controlling the contents of major alloying elements such as C, Si, Mn, and Alt, and strictly limiting the contents of impurity elements such as P and S, a good balance of high strength and high ductility is achieved. This optimization not only meets the tensile strength requirement of 590 MPa, but also ensures good formability and weldability.

[0083] (2) Metallographic structure control: By precisely controlling the volume fractions of ferrite and martensite, the microstructure of the material is optimized, thereby improving the overall performance of the material. Ferrite provides good plasticity and toughness, while martensite enhances the strength of the material.

[0084] (3) Process innovation: The adoption of process innovations such as direct heating, shortened transfer time, low-temperature furnace discharge, and shorter furnace time has effectively reduced production costs while maintaining high material quality. In addition, by optimizing the annealing process, such as precise control of heating, soaking, slow cooling, rapid cooling, and over-aging treatment steps, the yield strength and ductility of the material have been further improved.

[0085] (4) Production efficiency and cost control: By optimizing the process flow and parameter settings, production efficiency is improved, energy consumption and production costs are reduced. At the same time, shorter furnace time and low temperature furnace discharge also help reduce equipment wear and maintenance costs.

[0086] (5) High-quality products: The duplex steel products of this application have excellent mechanical properties and processing properties, and can meet the stringent requirements of the automotive, construction, machinery and other fields. In addition, the consistency and reliability of the products are ensured by precisely controlling the surface quality and internal structure.

[0087] (6) Environmentally friendly: By optimizing the alloy composition and process flow, the emission of harmful substances and the waste of resources are reduced, which meets the requirements of environmental protection and sustainable development.

[0088] The present application will be further described below in conjunction with specific examples. It should be understood that these examples are intended to illustrate the present application only and are not intended to limit the scope of the present application. The experimental methods in the following examples where specific conditions are not specified are generally measured according to industry standards. If there are no corresponding industry standards, then the methods are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0089] This embodiment provides a method for preparing high-elongation dual-phase steel for automobiles, which includes the following processes: a smelting production process, a hot rolling production process, and a continuous annealing process, wherein:

[0090] 1) The smelting production process includes: the molten steel composition is C: 0.07% to 0.09%, Si: 0.3% to 0.5%, Mn: 1.5% to 1.8%, Alt: 0.03% to 0.07%, P ≤ 0.002%, S ≤ 0.007%, and the balance is Fe and unavoidable impurities. The chemical composition of the smelted ingot is shown in Table 1;

[0091] 2) The hot rolling production process includes: billet charging - heating - holding - final rolling - coiling; wherein the billet charging adopts direct charging, and the charging temperature is required to be not less than 800°C, the billet discharge temperature is 1180°C to 1220°C, and the billet is held in the furnace for 160min to 180min. The final rolling temperature of the finishing rolling is 890±20°C, and the thickness of the hot-rolled steel strip is 2.5mm to 5.5mm. The cooling adopts the front-end intensive cooling mode with a cooling rate of 30±5°C / s. The coiling temperature is 530°C to 570°C. The specific process parameters of the hot rolling are shown in Table 2.

[0092] 3) The annealing process comprises: uncoiling the chilled steel strip after the pickling and cold rolling process and then heating it for continuous annealing, with the heating temperature being 780°C to 820°C, the soaking temperature being 780°C to 820°C, the slow cooling temperature being 580°C to 620°C, the rapid cooling temperature being 300°C to 330°C, the overaging temperature being 270°C to 310°C, the strip speed being 50m / min to 200m / min, and the flattening elongation being 0.2% to 0.5%. The specific process parameters of the continuous annealing are shown in Table 3.

[0093] Table 1: Chemical compositions (wt%) of Examples 1 to 3 of the present invention and the comparative example, the balance being Fe and unavoidable impurities

[0094] Example C Si Mn P S Alt 1 0.0836 0.4380 1.7200 0.0115 0.0030 0.0527 2 0.0830 0.4340 1.7000 0.0121 0.0024 0.0500 3 0.0743 0.4120 1.7000 0.0141 0.0040 0.0520 Comparative Example 0.091 0.2 2.02 0.014 0.002 0.75

[0095] Table 2: Hot rolling process parameters

[0096] Example Furnace charging temperature Oven temperature / ℃ Time in furnace / min Finish rolling temperature / ℃ Coiling temperature / ℃ 1 870 1202 163 894.46 586.68 2 850 1200 161 890.32 589.16 3 830 1205 171 890.75 589.07 Comparative Example 650 1250 155 859 522

[0097] Table 3: Hot dip galvanizing process parameters

[0098]

[0099] The mechanical properties of the dual-phase steels obtained in Examples 1 to 3 and the comparative example were measured, and the results are shown in Table 4.

[0100] Table 4: Mechanical properties

[0101] Yield strength MPa Tensile strength MPa Elongation % (A80) standard 340～390 ≥590 ≥27 Example 1 343 612 29 Example 2 348 614 28 Example 3 344 611 30 Comparative Example 394 682 23

[0102] The above data indicate that the present invention, through the adoption of a rationally proportioned composition design, high turnover efficiency, and reasonable hot-rolling and controlled-rolling and annealing processes, achieves a method for producing 590 MPa-grade low-cost, high-efficiency dual-phase steel for automotive use. The steel exhibits high elongation and a low yield ratio, and its mechanical properties meet both technical and user requirements, meeting the automotive industry's need for lightweighting. Furthermore, the present invention offers a low alloy cost and a simple preparation method, making it suitable for industrial production.

[0103] Figure 2 The metallographic structure diagram of the duplex steel provided in Example 1 of the present application; Figure 3 The metallographic structure diagram of the duplex steel provided for the comparative example of this application.

[0104] Depend on Figure 2 and Figure 3 It can be seen that Figure 2 Comparison Figure 3 The ferrite structure is coarser, the martensite proportion is smaller, the strength is reduced, and the elongation is increased.

[0105] In addition, one or more technical solutions in the embodiments of the present application have at least the following technical effects or advantages:

[0106] In the embodiments of the present application, on the basis of the existing process, the smelting cost is optimized by using less alloy and lower cost, the ingot is directly loaded after it comes off the line to improve the turnover efficiency, the hot rolling process adopts low temperature fast burning, and the annealing process adopts low temperature annealing, which ensures low cost and high efficiency in the production process, and finally obtains a low yield ratio and high ductility product, which meets the use requirements.

[0107] In the embodiment of the present application, the transfer time from slab to hot coil is shortened, the furnace is taken out at low temperature, and the time in the furnace is shorter, which not only meets the use requirements but also has low energy consumption, which is conducive to reducing production costs.

[0108] The foregoing is merely a list of specific embodiments of the present application, intended to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the broadest scope consistent with the principles and novel features of the present application.

Claims

1. A high ductility 590 MPa dual-phase steel, wherein the chemical composition of the dual-phase steel comprises, by mass fraction: C： 0.07%~0.09%, Si: 0.3%~0.5%, Mn: 1.5%~1.8%, Alt: 0.03%~0.07%, P≤0.002%, S≤0.007%, Fe.

2. The dual-phase steel according to claim 1, characterized in that Calculated by volume fraction, the metallographic structure of the dual-phase steel includes: ferrite: 85% to 95%, and martensite: 5% to 15%.

3. The dual-phase steel according to claim 1, characterized in that The dual-phase steel meets the following properties: yield strength of 340 MPa to 390 MPa, tensile strength ≥ 590 MPa, and elongation A80 ≥ 27%.

4. The dual-phase steel according to claim 1, characterized in that The thickness of the dual-phase steel is ≤2.6 mm.

5. A method for preparing the dual-phase steel according to any one of claims 1 to 4, comprising: Obtaining a casting billet having the chemical composition; The ingot is directly charged into a furnace at a set temperature for heating in the furnace, and then rolled to obtain a hot-rolled steel strip with a set thickness; The hot-rolled steel strip is cooled and coiled in a front-end intensive cooling mode to obtain a hot-rolled coil; Pickling and cold rolling the hot rolled coil to obtain a chilled steel strip; The chilled steel coil is continuously annealed and skin-passed to obtain the dual-phase steel.

6. The method according to claim 5, characterized in that The set temperature is ≥800°C, the holding time of the heating in the furnace is 160min-180min, and the outlet temperature of the heating in the furnace is 1180°C-1220°C.

7. The method according to claim 5, characterized in that The finishing temperature of the rolling is 870° C. to 910° C., and the set thickness is 2.5 mm to 5.5 mm.

8. The method according to claim 5, characterized in that The cooling speed is 25°C / s to 35°C / s, and the coiling temperature is 530°C to 570°C.

9. The method according to claim 5, characterized in that The step of continuously annealing and skin-passing the chilled steel strip to obtain the dual-phase steel comprises: Heating the chilled steel strip; the heating temperature is 780° C. to 820° C.; The heated chilled steel strip is subjected to uniform heating at a temperature of 780° C. to 820° C. Slowly cooling the chilled steel strip after soaking; the slow cooling temperature is 580° C. to 620° C.; The chilled steel strip after slow cooling is rapidly cooled; the rapid cooling temperature is 300° C. to 330° C.; The chilled steel strip after rapid cooling is subjected to an aging treatment; the temperature of the aging treatment is 270° C. to 310° C.; The over-aging treated chilled steel coil is skin-passed to obtain the dual-phase steel.

10. The method according to claim 9, characterized in that The belt speed of the continuous annealing is 50m / min to 200m / min, and the flattening elongation is 0.2% to 0.5%.

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