Low-cost high-surface cold-rolled dual-phase steel DP590 and manufacturing method thereof

Abstract

The invention relates to the technical field of dual-phase steel manufacturing. The invention discloses low-cost high-surface cold-rolled dual-phase steel DP590. The cold-rolled dual-phase steel DP590 comprises the following chemical components in percentage by mass: 0.08-0.1% of C, 0.3-0.5% of Si, 1.6-1.9% of Mn, less than or equal to 0.02% of P, less than or equal to 0.006% of S, 0.015-0.035% of Al, less than or equal to 0.006% of N and the balance of Fe and inevitable impurities. The invention further provides a manufacturing method of the low-cost high-surface cold-rolled dual-phase steel DP590. The manufacturing method sequentially comprises the following process steps of smelting, continuous casting, casting blank reheating, rough rolling, finish rolling, cooling and coiling after rolling, hot coil acid continuous rolling and rolled hard coil continuous annealing. The method has the beneficial effects that the alloy adding amount is reduced, the alloy cost is reduced, steelmaking casting, hot rolling and continuous annealing are all easy to control, and the manufacturing requirements for high strength, light weight and low cost of automobile parts are met.

Description

Technical Field

[0001] The present invention relates to the technical field of duplex steel manufacturing, and in particular to a low-cost, high-surface cold-rolled duplex steel DP590 and a manufacturing method thereof. Background Art

[0002] Dual-phase steel (DP steel) is a steel composed of a two-phase structure of martensite, austenite or bainite and a ferrite matrix. Generally, steel composed of ferrite and austenite phases is called duplex stainless steel, and steel composed of ferrite and martensite phases is called duplex steel. It can be obtained from low-carbon steel or low-alloy steel after critical zone treatment or controlled rolling. Duplex steel has a good combination of high strength and high ductility. It has become a new type of stamping steel with high strength and good formability, and is widely used in the automotive manufacturing industry. With the rapid development of the new energy vehicle industry, OEMs have put forward more stringent technical requirements for the high strength and lightweight of various components such as automotive reinforcements, anti-collision parts, battery pack guards, etc., hoping that duplex steel can achieve high performance while lowering manufacturing costs.

[0003] Chinese patent application publication number CN109112433A (publication date January 1, 2019) discloses a 590MPa grade cold-rolled dual-phase steel with no surface streak defects and a production method. The main chemical composition of the dual-phase steel is: C: 0.050% to 0.100%, Si: ≤0.25%, Mn: 1.20% to 2.00%, P: ≤0.012%, S: ≤0.008%, Al: 0.05% to 0.10%, Sb: 0.015% to 0.050%, and the balance is Fe and some other unavoidable impurities. This patent uses a low-Si content and Sb-added treatment method to treat the chemical composition. On a medium-thin slab continuous casting and rolling production line and a continuous annealing production line using the CAL cooling method, the 590MPa-grade cold-rolled duplex steel produced has a bright surface and is free of streak defects. However, the addition of the rare metal Sb will increase the alloy cost on the one hand, and Sb will easily aggregate and segregate at the grain boundaries, leading to grain boundary weakening on the other hand, causing surface warping defects in the strip during the hot rolling process.

[0004] Chinese patent application publication number CN118460924A (published on August 9, 2024) discloses a method for producing 590MPa-grade duplex steel with continuous annealing and galvanizing. The main chemical composition of the duplex steel is: C: 0.08%-0.10%, Si: 0.05%-0.09%, Mn: 1.40%-1.50%, Als: 0.8%-1.0%, Mo: 0.15%-0.20%, P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.005%, with the remainder being Fe and unavoidable impurities. This patent, while incorporating significant amounts of Al and precious metal Mo, increases the actual alloy cost. Furthermore, this increased Al addition increases the risk of nozzle nodules and contamination of the molten steel, as well as the risk of cracking in the ingot, leading to increased production costs.

[0005] Chinese patent application publication number CN109161805A (published on January 8, 2019) discloses a 590 MPa-grade lightweight cold-rolled dual-phase steel for automotive use and its production method. The chemical composition of the dual-phase steel is as follows: C: 0.06-0.08%, Si: 0.05-0.10%, Mn: 1.70-1.80%, P: ≤0.010%, S: ≤0.005%, Alt: 0.020-0.050%, Cr: 0.20-0.30%. This patent utilizes a C-Si-Mn-Cr chemical composition, requiring the addition of 0.2-0.3% Cr. This increases alloy costs and fails to meet the requirements for high-performance, low-cost dual-phase steel production.

[0006] Chinese patent application publication number CN115181883A (publication date October 14, 2022) discloses a 590MPa-grade low-carbon, low-alloy, high-formability dual-phase steel and a rapid heat treatment manufacturing method. The chemical composition of the dual-phase steel is as follows: C: 0.04-0.10%, Si: 0.1-0.3%, Mn: 1.0-1.6%, P≤0.02%, S≤0.015%, Al: 0.02-0.06%, and it also contains one or two of Cr, Mo, Ti, Nb, and V, with Cr+Mo+Ti+Nb+V≤0.5%, and the balance is Fe and other unavoidable impurities. By controlling the rapid heating, short-term holding, and rapid cooling processes during the rapid heat treatment process, the patent changes the recovery, recrystallization, and austenite transformation processes of the deformed structure, increases the nucleation rate (including the recrystallization nucleation rate and the austenite phase deformation nucleation rate), shortens the grain growth time, refines the grains, and improves the strength and n value of the material. However, in order to achieve rapid heating and cooling, complex equipment upgrades and modifications are required, which significantly increases manufacturing costs. In addition, it requires alloy elements such as Cr, Mo, Ti, Nb, and V, and the alloy cost is relatively high.

[0007] Therefore, in response to the above existing problems, how to stably provide a low-cost, excellent mechanical properties, good surface quality and easy-to-manufacture duplex steel has become a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0008] The present invention aims to provide a low-cost, high-surface cold-rolled dual-phase steel DP590 and a method for manufacturing the same. The method reduces alloy additions and alloy costs while also facilitating control of steelmaking, casting, hot rolling, and continuous annealing, thereby meeting the high-strength, lightweight, and low-cost manufacturing requirements for automotive parts.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions: Disclosed is a low-cost, high-surface cold-rolled dual-phase steel DP590. The chemical composition of the cold-rolled dual-phase steel DP590 is as follows, by mass percentage: C: 0.08-0.10%, Si: 0.30-0.50%, Mn: 1.60-1.90%, P≤0.02%, S≤0.006%, Al: 0.015-0.035%, N≤0.006%, and the balance is Fe and unavoidable impurities.

[0010] Preferably, the thickness specification of the cold-rolled dual-phase steel DP590 is 0.6-2.5 mm.

[0011] Preferably, the microstructure of the cold-rolled dual-phase steel DP590 is ferrite and martensite, and the grain size grade is ≥10.

[0012] Preferably, the yield strength R of the cold-rolled dual-phase steel DP590 in the 90° direction is t0.2 350~420MPa, tensile strength R m ≥590MPa, elongation at break A 80mm ≥20%, yield ratio ≤0.7, the cold-rolled dual-phase steel DP590 shows no cracks in a 180°d=0t cold bending test, where d is the bending center diameter and t is the thickness of the finished steel plate.

[0013] The present invention also provides a method for manufacturing low-cost high-surface cold-rolled dual-phase steel DP590, the manufacturing method comprising the following process steps performed in sequence: smelting, continuous casting, reheating of cast billets, rough rolling, finish rolling, post-rolling cooling and coiling, hot coil acid continuous rolling, and continuous annealing of hard coils; Wherein, during the reheating stage of the cast slab, the furnace temperature is controlled to be 1170-1260°C; During the rough rolling stage and the finishing rolling stage, the dephosphorization water pressure is controlled to be ≥220 MPa; In the finishing rolling stage, the final rolling temperature is controlled to be 870-940°C; During the post-rolling cooling and coiling stages, the coiling temperature is controlled to be 550-650°C; During the hot coil pickling continuous rolling stage, the cold rolling reduction ratio is controlled to be ≥44%; In the continuous annealing stage of the hard rolled coil, the strip is heated to 770-790°C at a heating rate of 5-10°C / s, kept at this temperature for 90-110s, and then slowly cooled to 600-620°C at a cooling rate of 10-15°C / s, then rapidly cooled to 300-330°C at a cooling rate of 40-45°C / s, and then subjected to overaging treatment for 300-320s. The flattening elongation is 0.7-1.1%.

[0014] Preferably, the smelting process includes KR stirring pre-desulfurization, LD top and bottom composite blowing, and RH degassing.

[0015] Preferably, the continuous casting stage includes full-process protective pouring, full-process dynamic secondary cooling control, and full-process soft reduction control.

[0016] Preferably, during the continuous casting stage, the thickness of the ingot is controlled to be 230 mm.

[0017] Preferably, in the post-rolling cooling and coiling stage, the strip is rapidly cooled to 560-620° C. at a cooling rate of 25-35° C. / s immediately after the final pass of finishing rolling, and medium-low temperature coiling is performed.

[0018] Preferably, during the continuous annealing stage of the hard rolled coil, a high hydrogen rapid cooling process is used to achieve rapid cooling.

[0019] Beneficial effects of the present invention: 1. Compared with the prior art, the chemical composition of the cold-rolled dual-phase steel DP590 provided by the present invention is mainly composed of C, Si, and Mn, and no alloying elements such as Cr, Nb, Ti, V, Mo, and Sb are added. While reducing alloy costs, it can also produce easy-to-cast, high-purity, low-segregation, and crack-free ingots, which is conducive to the realization of high-efficiency hot delivery and hot charging.

[0020] 2. The manufacturing method provided by the present invention is to specifically control the dephosphorization water pressure in the ingot reheating stage and the hot rolling stage, as well as the precise cooling system after finishing rolling, so as to stably obtain hot-rolled strip steel with less surface oxide scale, uniform oxide scale, uniform temperature and performance, and then complete continuous heat treatment of the strip steel in a high hydrogen atmosphere, so as to stably obtain finished strip steel with less surface element enrichment, excellent mechanical properties and good surface quality, which effectively meets the use requirements of automotive parts.

[0021] 3. The yield strength R of the cold-rolled dual-phase steel DP590 provided by the present invention in the 90° direction t0.2 350~420MPa, tensile strength R m≥590MPa, elongation at break A 80mm ≥20%, yield strength ratio ≤0.7, with excellent mechanical properties, meeting the needs of high-strength steel.

[0022] 4. The cold-rolled dual-phase steel DP590 provided by the present invention has excellent cold bending forming performance. It can be cold-bent 180° with a bending center diameter d=0t without cracks on the steel plate surface. It can meet the cold bending and blanking requirements in the processing of automotive parts and ensure that cracks will not occur during the processing of automotive parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a statistical table of chemical composition parameters of Examples 1-8 of the present invention.

[0024] Figure 2 This is a statistical table of hot rolling process parameters for Examples 1-8 of the present invention.

[0025] Figure 3 This is a statistical table of cold rolling annealing process parameters for Examples 1-8 of the present invention.

[0026] Figure 4 This is a statistical table of mechanical property parameters of the finished rolls of Examples 1-8 of the present invention in the 90° direction.

[0027] Figure 5 This is a typical metallographic photograph of Example 1 of the present invention.

[0028] Figure 6 This is a typical scanning electron microscope photograph of Example 1 of the present invention.

[0029] Figure 7 This is a typical surface photograph of Example 1 of the present invention. DETAILED DESCRIPTION

[0030] The present invention will be described in detail below in conjunction with specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly presented. It should be understood by those skilled in the art that these specific embodiments and examples are for illustrating the present invention, rather than for limiting the present invention.

[0031] According to a typical embodiment of the present invention, a low-cost, high-surface cold-rolled dual-phase steel DP590 is provided. The chemical composition of the cold-rolled dual-phase steel DP590 is as follows, by mass percentage: C: 0.08-0.10%, Si: 0.30-0.50%, Mn: 1.60-1.90%, P ≤ 0.02%, S ≤ 0.006%, Al: 0.015-0.035%, N ≤ 0.006%, and the balance is Fe and unavoidable impurities.

[0032] The main functions and design basis of the chemical elements corresponding to the cold-rolled dual-phase steel DP590 described in the present invention are as follows: Carbon (C) is the most fundamental and economical strengthening element in steel. It also primarily influences the martensite, ferrite, and retained austenite content, as well as the performance, of duplex steel. If the C content is too low, more alloying must be added to improve the steel's mechanical properties. However, if the C content is too high, the steel becomes brittle and its weldability deteriorates. Therefore, the C content in this invention is controlled to 0.08-0.10%.

[0033] Si: A non-carbide-forming element, it increases the austenite-to-ferrite transition temperature and promotes ferrite precipitation. It also provides solid solution strengthening and deoxidizes the molten steel. However, excessive Si content can easily lead to surface quality issues such as black spots on the surface of hot-rolled ironscale and even streaks in the finished product after continuous annealing, and can also reduce the weldability of the steel. Therefore, the Si content in this invention is controlled to 0.30-0.50%.

[0034] Mn: A key strengthening element in the present invention, it lowers the austenite-to-ferrite transformation temperature, refines the ferrite grain size, and alters the microstructure after the transformation. Too low a Mn content reduces the steel's strength, plasticity, and toughness. Too high a Mn content increases the difficulty of controlling centerline segregation and surface precipitation during annealing, and reduces the steel's weldability. Therefore, the Mn content in the present invention is controlled to 1.60-1.90%.

[0035] Al: In this invention, it serves solely as a deoxidizer, removing dissolved oxygen from the steel to ensure molten steel purity. Therefore, the Al content in this invention is controlled to 0.015-0.035%. To maximize Al's role in regulating phase transformation and enhancing toughness, a higher Al content would be required. This would increase alloy costs and require specialized high-aluminum mold slag for continuous steelmaking to prevent cracking in the ingots, increasing manufacturing complexity.

[0036] P, S, and N: These are all harmful elements in steel and must be carefully controlled to avoid quality defects such as hot brittleness, cold brittleness, and delayed brittleness. Therefore, in the present invention, the P content is controlled to ≤0.020%, the S content is controlled to ≤0.006%, and the N content is controlled to ≤0.006%.

[0037] As a preferred embodiment, the thickness specification of the cold-rolled dual-phase steel DP590 is 0.6-2.5 mm.

[0038] As a preferred embodiment, the microstructure of the cold-rolled dual-phase steel DP590 is ferrite and martensite, and the grain size grade is ≥10.

[0039] As a preferred embodiment, the yield strength R of the cold-rolled dual-phase steel DP590 in the 90° direction is t0.2 350~420MPa, tensile strength R m ≥590MPa, elongation at break A 80mm ≥20%, yield ratio ≤0.7, the cold-rolled dual-phase steel DP590 shows no cracks in a 180°d=0t cold bending test, where d is the bending center diameter and t is the thickness of the finished steel plate.

[0040] The present invention also provides a method for manufacturing low-cost high-surface cold-rolled dual-phase steel DP590, the manufacturing method comprising the following process steps performed in sequence: smelting, continuous casting, reheating of cast billets, rough rolling, finish rolling, post-rolling cooling and coiling, hot coil acid continuous rolling, and continuous annealing of hard coils; Wherein, during the reheating stage of the cast slab, the furnace temperature is controlled to be 1170-1260°C; During the rough rolling stage and the finishing rolling stage, the dephosphorization water pressure is controlled to be ≥220 MPa; In the finishing rolling stage, the final rolling temperature is controlled to be 870-940°C; During the post-rolling cooling and coiling stages, the coiling temperature is controlled to be 550-650°C; During the hot coil pickling continuous rolling stage, the cold rolling reduction ratio is controlled to be ≥44%; In the continuous annealing stage of the hard rolled coil, the strip is heated to 770-790°C at a heating rate of 5-10°C / s, kept at this temperature for 90-110s, and then slowly cooled to 600-620°C at a cooling rate of 10-15°C / s, then rapidly cooled to 300-330°C at a cooling rate of 40-45°C / s, and then subjected to overaging treatment for 300-320s. The flattening elongation is 0.7-1.1%.

[0041] As a preferred embodiment, the smelting process includes KR stirring pre-desulfurization, LD top and bottom composite blowing, and RH degassing.

[0042] As a preferred embodiment, the continuous casting stage includes full-process protective pouring, full-process dynamic secondary cooling control, and full-process soft reduction control.

[0043] As a preferred embodiment, during the continuous casting stage, the thickness of the ingot is controlled to be 230 mm.

[0044] As a preferred embodiment, in the post-rolling cooling and coiling stage, the strip is rapidly cooled to 560-620°C at a cooling rate of 25-35°C / s immediately after the final pass of finishing rolling, and medium-low temperature coiling is performed.

[0045] As a preferred embodiment, during the continuous annealing stage of the hard rolled coil, a high hydrogen rapid cooling process is used to achieve rapid cooling.

[0046] It should be noted that, in the present method, full-process protective casting refers to an important technical measure to protect the molten steel exposed to the air during the continuous casting process to avoid secondary oxidation of the molten steel by the air; full-process dynamic secondary cooling control refers to dynamic cooling control of the billet in the secondary cooling zone of continuous casting to improve cooling uniformity and ensure the quality of the billet. It is an important technical measure; full-process light reduction control refers to applying pressure to the billet by changing the roll gap near the solidification end of the continuous casting billet to produce a certain amount of reduction to compensate for the solidification shrinkage of the billet, which is an important technical measure for improving the central porosity and central segregation of the billet; therefore, after the smelting and continuous casting stages, billets that are easy to cast, have high purity, low segregation, and no crack defects can be obtained, which is also conducive to the realization of high-efficiency hot delivery and hot loading.

[0047] It should be understood that the furnace temperature is controlled at 1170-1260℃ and the dephosphorization water pressure is controlled at ≥220MPa, which can minimize or eliminate the influence of Si-containing red iron scale and obtain hot-rolled strip with less surface oxide scale, uniform oxide scale, uniform temperature and performance.

[0048] It should be understood that when the final rolling temperature in the finishing rolling stage is controlled at 890-920°C, the strip is immediately cooled to 560-620°C at a cooling rate of 25-35°C / s after the last pass of finishing rolling, and medium-low temperature coiling is performed; in this process, due to the introduction of accelerated cooling, the austenite to ferrite phase transformation temperature is reduced, the supercooling is increased, and the nucleation rate of ferrite is increased; at the same time, the premature precipitation of carbon / nitrides during the cooling process is delayed, so that more dispersed precipitates are easily generated, further refining the ferrite grains; on the one hand, it can improve the temperature and performance uniformity of the hot-rolled strip, and on the other hand, it can reduce the production difficulty of hot-rolled acid continuous rolling.

[0049] It should be understood that in the hot coil acid rolling stage, the cold rolling reduction rate is controlled at ≥44%. In the continuous annealing stage of hard coil rolling, the strip undergoes partial austenitization after high-temperature soaking and heat preservation at 770-790°C, and slowly cools to 600-620°C, which can make a small amount of austenite decompose and transform into ferrite. Subsequently, a high-hydrogen rapid cooling process is adopted to rapidly cool to 300-330°C, and the retained austenite is transformed into martensite to obtain a dual-phase steel structure. Then, over-aging treatment and flattening are carried out to improve the solid solution state of elements in the ferrite and the surface quality of the strip.

[0050] It should be noted that the cold-rolled dual-phase steel DP590 described in the present invention is designed based on specific part application targets. Different part production requires different mechanical properties and cold bending requirements for dual-phase steel. Therefore, refined process control is essential for controlling the strength grade required by the present invention. To stably produce steel that meets the strength-ductility balance and cold bending requirements of the corresponding parts, the heating temperature must be strictly limited and controlled to ensure sufficient and uniform austenitization. Controlling the cooling start temperature affects the actual ferrite content, while the cooling rate is the most critical factor for dual-phase steel. Controlling the cooling rate is the only way to suppress non-martensitic phase transformations. Slight fluctuations in chemical composition and process parameters have a significant impact on the performance of dual-phase steel, especially in terms of strength and cold bending, where the actual process window is narrower and the control requirements are more stringent. Only through refined process settings and control can a microstructure and final performance that meet the target requirements be achieved based on the designed chemical composition.

[0051] The low-cost, high-surface cold-rolled dual-phase steel DP590 and its manufacturing method provided by the present invention will be described in detail below with reference to examples and experimental data.

[0052] The manufacturing methods of low-cost, high-surface cold-rolled dual-phase steel DP590 of Examples 1-8 were established, and the manufacturing process flows were: KR method stirring pre-desulfurization → LD top and bottom composite blowing → RH degassing → continuous casting → ingot reheating → rough rolling → finishing rolling → post-rolling cooling and coiling → hot coil acid continuous rolling → hard coil continuous annealing.

[0053] The chemical composition of the low-cost high-surface cold-rolled dual-phase steel DP590 of Examples 1-8 is as follows: Figure 1 As shown; the hot rolling process parameters of Examples 1-8 are as follows Figure 2 As shown; the cold rolling annealing process parameters of Examples 1-8 are as follows Figure 3 The mechanical properties of the finished rolls of Examples 1-8 in the 90° direction are shown as follows: Figure 4 shown.

[0054] Depend on Figure 4 It can be seen that the yield strength R of the cold-rolled dual-phase steel DP590 provided by the present invention in the 90° direction is t0.2 350~420MPa, tensile strength R m ≥590MPa, elongation at break A 80mm ≥20%, yield strength ratio ≤0.7. At the same time, the finished coil has excellent cold bending forming performance. It can be cold bent 180° with a bending core diameter of d=0t. There is no crack on the steel plate surface, which can meet the cold bending and blanking requirements in the processing of automotive parts and ensure that there will be no cracking during the processing of automotive parts.

[0055] Please refer to Figure 5 and Figure 6 , Figure 5and Figure 6 These are typical metallographic photos and typical scanning electron microscope photos of Example 1, respectively. It can be clearly observed that the microstructure of the cold-rolled dual-phase steel DP590 provided by the present invention is mainly ferrite and martensite.

[0056] Please refer to Figure 7 , Figure 7 This is a typical surface photo of Example 1. It can be clearly observed that the surface of the steel coil has no quality defects such as stripes and color difference, and the surface quality is excellent.

Claims

1. A low-cost, high-surface cold-rolled dual-phase steel DP590, characterized in that: The chemical composition of the cold-rolled dual-phase steel DP590 is as follows by mass percentage: C: 0.08-0.10%, Si: 0.30-0.50%, Mn: 1.60-1.90%, P≤0.02%, S≤0.006%, Al: 0.015-0.035%, N≤0.006%, and the balance is Fe and unavoidable impurities.

2. The low-cost, high-surface cold-rolled dual-phase steel DP590 according to claim 1, characterized in that: The thickness specification of the cold-rolled dual-phase steel DP590 is 0.6-2.5 mm.

3. The low-cost, high-surface cold-rolled dual-phase steel DP590 according to claim 1, characterized in that: The microstructure of the cold-rolled dual-phase steel DP590 is ferrite and martensite, and the grain size grade is ≥10.

4. The low-cost, high-surface cold-rolled dual-phase steel DP590 according to claim 1, characterized in that: The yield strength R of the cold-rolled dual-phase steel DP590 in the 90° direction is t0.2 350~420MPa, tensile strength R m ≥590MPa, elongation at break A 80mm ≥20%, yield ratio ≤0.7, the cold-rolled dual-phase steel DP590 shows no cracks in a 180°d=0t cold bending test, where d is the bending center diameter and t is the thickness of the finished steel plate.

5. A method for manufacturing low-cost, high-surface cold-rolled dual-phase steel DP590 according to any one of claims 1 to 4, characterized in that: The manufacturing method comprises the following process steps carried out in sequence: smelting, continuous casting, reheating of cast billets, rough rolling, finishing rolling, cooling and coiling after rolling, hot coil acid continuous rolling, and continuous annealing of hard coils; Wherein, during the reheating stage of the cast slab, the furnace temperature is controlled to be 1170-1260°C; During the rough rolling stage and the finishing rolling stage, the dephosphorization water pressure is controlled to be ≥220 MPa; In the finishing rolling stage, the final rolling temperature is controlled to be 870-940°C; During the post-rolling cooling and coiling stages, the coiling temperature is controlled to be 550-650°C; During the hot coil pickling continuous rolling stage, the cold rolling reduction ratio is controlled to be ≥44%; In the continuous annealing stage of the hard rolled coil, the strip is heated to 770-790°C at a heating rate of 5-10°C / s, kept at this temperature for 90-110s, and then slowly cooled to 600-620°C at a cooling rate of 10-15°C / s, then rapidly cooled to 300-330°C at a cooling rate of 40-45°C / s, and then subjected to overaging treatment for 300-320s. The flattening elongation is 0.7-1.1%.

6. The method for manufacturing low-cost, high-surface cold-rolled dual-phase steel DP590 according to claim 5, characterized in that: The smelting process includes KR stirring pre-desulfurization, LD top and bottom composite blowing, and RH degassing.

7. The method for manufacturing low-cost, high-surface cold-rolled dual-phase steel DP590 according to claim 5, characterized in that: The continuous casting stage includes full-process protective pouring, full-process dynamic secondary cooling control, and full-process soft reduction control.

8. The method for manufacturing low-cost, high-surface cold-rolled dual-phase steel DP590 according to claim 7, characterized in that: During the continuous casting stage, the thickness of the ingot is controlled to be 230 mm.

9. The method for manufacturing low-cost, high-surface cold-rolled dual-phase steel DP590 according to claim 5, characterized in that: In the post-rolling cooling and coiling stage, the strip is immediately cooled to 560-620° C. at a cooling rate of 25-35° C. / s after the final finishing rolling, and is then coiled at a medium-low temperature.

10. The method for manufacturing low-cost, high-surface cold-rolled dual-phase steel DP590 according to claim 5, characterized in that: During the continuous annealing stage of the hard rolled coil, a high hydrogen rapid cooling process is adopted to achieve rapid cooling.

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