Galvanized dual-phase steel of different yield strength grades and 780 mpa tensile strength grade and manufacturing method therefor

Abstract

The present application discloses a galvanized dual-phase steel of different yield strength grades and 780 MPa tensile strength grade and a manufacturing method therefor. The manufacturing method comprises: providing a continuous casting slab, the continuous casting slab comprising, in mass percentage, the following components: C: 0.06%-0.09%, Si: 0.10%-0.25%, Mn: 1.9%-2.3%, Als: 0.40%-0.70%, Nb: 0.01%-0.025%, Cr: 0.22%-0.30%, Mo: 0.15%-0.20%, P≤0.015%, S≤0.025%, N≤0.005%, and the balance being Fe and unavoidable impurities; subjecting the continuous casting slab to hot rolling treatment at 1220°C to 1290°C to obtain a hot-rolled steel coil, the finish rolling temperature in the hot rolling treatment being 850°C to 930°C and the coiling temperature being 520°C to 640°C; subjecting the hot-rolled steel coil to pickling treatment to obtain a pickled steel coil; subjecting the pickled steel coil to cold rolling to obtain a hard-rolled coil, the total reduction ratio of the cold rolling being 55% to 67%; and subjecting the hard-rolled coil to annealing and galvanizing treatment to obtain a galvanized steel sheet, the annealing temperature being 755°C to 770°C or 775°C to 790°C. The manufacturing method for the galvanized dual-phase steel is capable of producing galvanized dual-phase steel having a yield strength of 420 MPa to 550 MPa or 500 MPa to 650 MPa and a tensile strength of ≥780 MPa using a continuous casting slab of the same composition system.

Description

Galvanized duplex steels with different yield strength grades and 780MPa tensile strength grades and their manufacturing methods Technical Field

[0001] This application belongs to the field of metallurgical technology, and in particular relates to a galvanized duplex steel with different yield strength levels and a tensile strength level of 780MPa and its manufacturing method. Background Technology

[0002] Galvanized duplex steel, as a material with a good overall performance and cost-effectiveness in automotive structural components, boasts advantages such as excellent corrosion resistance, high initial work hardening rate, high impact energy absorption capacity, good weldability, and a good balance of strength and ductility, leading to its increasing application in automobiles. Simultaneously, with the development of lightweight vehicles, increasingly stringent user requirements for corrosion resistance, and environmental challenges facing the automotive industry, lightweighting has become a crucial development strategy. According to relevant research, under the premise of ensuring passenger safety, every 10% reduction in vehicle weight leads to approximately 8% reduction in fuel consumption and 4% reduction in emissions. Therefore, lightweighting is the optimal means of energy conservation and emission reduction. However, ensuring passenger safety while simultaneously reducing the thickness and weight of steel plates necessitates increasing the strength of the steel plates. Currently, the application of galvanized duplex steel with a tensile strength of 780MPa is becoming increasingly widespread in automobiles. With the rapid development of new energy vehicles, the requirements for weight reduction are even higher than for gasoline vehicles, leading to the extensive use of galvanized duplex steel with a tensile strength of 780MPa in structural components, reinforcements, and battery packs. The strength requirements of the material vary depending on the application scenario. Taking the battery pack of new energy vehicles as an example, this component generally does not require large stamping deformation during use. While ensuring formability, it should have as high a yield strength and tensile strength as possible to reduce deformation of the battery pack under impact. Structural components and reinforcements, compared to the battery pack, have a larger amount of stamping deformation. While ensuring strength, they need lower yield strength, higher elongation, and greater porosity to reduce stamping cracking. Therefore, it is necessary to develop galvanized duplex steel with different yield strength levels and tensile strength levels of 780MPa to meet user needs.

[0003] However, due to production organization issues, it is difficult for enterprises to simultaneously meet the requirements of multi-performance and rapid production of galvanized duplex steel with different yield strength grades and 780MPa tensile strength grades. Summary of the Invention

[0004] This application provides a method for manufacturing galvanized duplex steel with different yield strength levels and a tensile strength level of 780MPa. It can produce galvanized duplex steel with yield strengths of 420MPa to 550MPa and 500MPa to 650MPa, and tensile strengths of ≥780MPa using continuously cast slabs with the same composition system.

[0005] In a first aspect, this application provides a method for manufacturing galvanized duplex steel with different yield strength levels and a tensile strength level of 780 MPa, comprising: providing a continuously cast slab, wherein the continuously cast slab comprises the following components by mass percentage: C: 0.06%~0.09%, Si: 0.10%~0.25%, Mn: 1.9%~2.3%, Als: 0.40%~0.70%, Nb: 0.01%~0.025%, Cr: 0.22%~0.30%, Mo: 0.15%~0.20%, P≤0.015%, S≤0.025%, N≤0. The steel contains 0.005% Fe and unavoidable impurities; the continuously cast slab is hot-rolled at 1220℃~1290℃ to obtain hot-rolled steel coils; the finishing rolling temperature in the hot rolling process is 850℃~930℃, and the coiling temperature is 520℃~640℃; the hot-rolled steel coils are pickled to obtain pickled steel coils; the pickled steel coils are cold-rolled to obtain cold-hardened coils; the total reduction rate of the cold rolling process is 55%~67%; the cold-hardened coils are annealed and galvanized to obtain galvanized steel sheets; the annealing temperature is 755℃~770℃ or 775℃~790℃.

[0006] According to an embodiment of the first aspect of this application, providing a continuously cast slab includes: obtaining it by smelting according to the chemical composition of the continuously cast slab.

[0007] According to an embodiment of the first aspect of this application, the thickness of the continuously cast slab is 220 mm to 260 mm.

[0008] According to an embodiment of the first aspect of this application, hot rolling of a continuously cast slab at 1220°C to 1290°C includes: heating the continuously cast slab to 1220°C to 1290°C; subjecting the continuously cast slab heated to 1220°C to 1290°C to rough rolling and finish rolling processes in sequence to obtain hot-rolled steel; and coiling the hot-rolled steel to obtain hot-rolled steel coils.

[0009] According to the embodiment of the first aspect of this application, the heating time for heating the continuously cast slab at 1220°C to 1290°C is 150 minutes to 400 minutes.

[0010] According to an embodiment of the first aspect of this application, the thickness of the hot-rolled steel coil is 2.4 mm to 4.5 mm.

[0011] According to the embodiments of the first aspect of this application, the pickling temperature of the hot-rolled steel coil is ≥75°C and the acid concentration is ≥100g / L.

[0012] According to an embodiment of the first aspect of this application, the manufacturing method of galvanized duplex steel with different yield strength levels of 780MPa further includes: performing finishing and tension leveling treatment on the galvanized steel sheet, wherein the finishing elongation of the finishing treatment on the galvanized steel sheet is 0.3% to 0.6%, the finishing rolling force is ≥200T, and the tension leveling elongation of the galvanized steel sheet is 0.05% to 0.15%.

[0013] According to an embodiment of the first aspect of this application, manufacturing galvanized duplex steel with yield strength of 420MPa to 550MPa and tensile strength of 780MPa includes: hot rolling a continuously cast slab at 1220°C to 1290°C to obtain a hot-rolled steel coil; wherein the finishing rolling temperature in the hot rolling process is 850°C to 930°C and the coiling temperature is 560°C to 640°C.

[0014] According to an embodiment of the first aspect of this application, manufacturing galvanized duplex steel with yield strength of 500MPa to 650MPa and tensile strength of 780MPa includes: hot rolling a continuously cast slab at 1220°C to 1290°C to obtain a hot-rolled steel coil; wherein the finishing rolling temperature in the hot rolling process is 850°C to 930°C and the coiling temperature is 520°C to 600°C.

[0015] Secondly, this application provides a galvanized duplex steel, which is manufactured according to the above-mentioned manufacturing method of galvanized duplex steel with different yield strength levels and 780MPa tensile strength level. The galvanized duplex steels are respectively galvanized duplex steel with yield strength of 420MPa to 550MPa, tensile strength ≥780MPa, and elongation ≥17%; and galvanized duplex steel with yield strength of 500MPa to 650MPa, tensile strength ≥780MPa, and elongation ≥14%.

[0016] According to an embodiment of the second aspect of this application, the thickness of the galvanized duplex steel is 0.8 mm to 2.0 mm.

[0017] The manufacturing methods of galvanized duplex steel with different yield strength levels and 780MPa tensile strength levels in this application embodiment can produce galvanized duplex steel with yield strengths of 420MPa~550MPa and 500MPa~650MPa, and tensile strengths ≥780MPa using continuously cast slabs of the same composition system. This solves the problem of simultaneously achieving multiple properties and rapid production of galvanized duplex steel, meets the requirements of production organization, reduces the production cost of enterprises, and ensures the requirements of multiple performance levels of the product. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 is a schematic flowchart of the manufacturing methods of galvanized duplex steel with different yield strength levels and 780MPa tensile strength levels provided in the embodiments of this application.

[0020] Figure 2 is a metallographic diagram of the galvanized duplex steel with a yield strength of 420 MPa and a tensile strength of 780 MPa provided in Embodiment 3 of this application.

[0021] Figure 3 is a metallographic diagram of the galvanized duplex steel with a yield strength of 500 MPa and a tensile strength of 780 MPa provided in Embodiment 7 of this application. Detailed Implementation

[0022] The features and exemplary embodiments of various aspects of this application will be described in detail below. To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain this application and not to limit it. For those skilled in the art, this application can be implemented without some of these specific details. The following description of the embodiments is merely to provide a better understanding of this application by illustrating examples.

[0023] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes said element.

[0024] The inventors of this application have discovered that by rationally designing the composition of galvanized duplex steel and using controlled rolling and cooling processes, a suitable martensitic and ferrite dual-phase structure can be obtained to ensure strength. By controlling the martensite content in galvanized duplex steel with a tensile strength of 780 MPa, different yield strength levels of galvanized duplex steel can be obtained. Based on this principle, a manufacturing method for producing galvanized duplex steel with different yield strength levels and 780 MPa tensile strength levels can be developed.

[0025] To address the problems of the prior art, this application provides galvanized duplex steel with different yield strength levels and a tensile strength level of 780 MPa, and a method for manufacturing the same. The manufacturing method of the galvanized duplex steel with different yield strength levels and a tensile strength level of 780 MPa provided in this application will be described below.

[0026] Figure 1 shows a schematic flowchart of the manufacturing methods of galvanized duplex steel with different yield strength levels and 780MPa tensile strength levels provided in the embodiments of this application.

[0027] As shown in Figure 1, the manufacturing method of galvanized duplex steel with different yield strength grades and 780 MPa tensile strength grades includes: providing a continuously cast slab, which, by mass percentage, comprises the following components: C: 0.06%~0.09%, Si: 0.10%~0.25%, Mn: 1.9%~2.3%, Als: 0.40%~0.70%, Nb: 0.01%~0.025%, Cr: 0.22%~0.30%, Mo: 0.15%~0.20%, P≤0.015%, S≤0.025%, N≤0.00%. 5%, the remainder being Fe and unavoidable impurities; the continuously cast slab is hot-rolled at 1220℃~1290℃ to obtain hot-rolled steel coils; wherein, the finishing rolling temperature in the hot rolling process is 850℃~930℃, and the coiling temperature is 560℃~640℃; the hot-rolled steel coils are pickled to obtain pickled steel coils; the pickled steel coils are cold-rolled to obtain cold-hardened coils; wherein, the total reduction rate of cold rolling is 55%~67%; the cold-hardened coils are annealed and galvanized to obtain galvanized steel sheets; wherein, the annealing temperature is 755℃~770℃ or 775℃~790℃.

[0028] The manufacturing method of galvanized duplex steel with different yield strength levels and a tensile strength level of 780MPa in this application embodiment, by effectively controlling the key process parameters of each process, enables the production of galvanized duplex steel with two different yield strengths of 420MPa~550MPa and 500MPa~650MPa, and a tensile strength of ≥780MPa, using the same composition system and sequentially through steelmaking to provide continuously cast slabs, hot rolling, pickling, cold rolling and galvanizing processes. This method can meet the requirements of production organization, reduce the production cost of enterprises, and ensure the multi-level performance requirements of the product.

[0029] This application describes a method for manufacturing galvanized duplex steel with different yield strength levels and 780MPa tensile strength levels. Developing galvanized duplex steel with different yield strength levels and 780MPa tensile strength levels can meet different stamping requirements, solve production organization problems for enterprises, ensure delivery time, and reduce unnecessary losses. Furthermore, since producing galvanized duplex steel with a tensile strength of 780MPa requires high equipment capacity, technical level, and on-site production stability control, this product offers significant economic benefits. Therefore, using continuously cast slabs with the same composition system to produce galvanized duplex steel with two or more yield strength levels and 780MPa tensile strength levels not only facilitates rapid production organization and reduces production costs for steel production enterprises, bringing certain economic benefits, but also meets the automotive industry's requirements for multiple parts, multiple performance characteristics, and fast delivery times, thus solving the problem of simultaneously achieving multiple performance characteristics and rapid production of galvanized duplex steel.

[0030] The manufacturing methods of galvanized duplex steel with different yield strength levels and 780MPa tensile strength levels in this application improve the hardenability of austenite during critical zone annealing by rationally selecting the chemical element composition and its content, especially by controlling the reasonable content of C, Si, Mn, Cr, Al, Mo, and Nb in the galvanized duplex steel, ensuring the martensite content in the microstructure of the galvanized duplex steel, and controlling the content of S and N to extremely low levels, thus ensuring that the galvanized duplex steel has good mechanical properties and formability.

[0031] The manufacturing methods of galvanized duplex steel with different yield strength levels and 780MPa tensile strength levels in this application embodiment adopt low carbon composition and add low cost alloys such as Si and Mn, and add appropriate Al, Nb, Cr and Mo alloys to ensure product performance and process feasibility.

[0032] In the embodiments of this application, the composition of the continuously cast slab obtained from steelmaking, the hot rolling treatment and the coiling temperature after rolling, the pickling treatment, the cold rolling and galvanizing treatment play an important role in obtaining galvanized duplex steel with different yield strength levels and 780MPa tensile strength levels.

[0033] In some embodiments of this application, providing a continuously cast slab includes obtaining it by smelting according to the chemical composition of the continuously cast slab. The smelting can be carried out according to smelting methods known in the art to obtain the continuously cast slab.

[0034] In some embodiments of this application, the thickness of the continuously cast slab is 220 mm to 260 mm.

[0035] In some embodiments of this application, the heating time for heating the continuously cast slab at 1220℃~1290℃ is 150 minutes to 400 minutes.

[0036] In some embodiments of this application, the thickness of the hot-rolled steel coil is 2.4 mm to 4.5 mm.

[0037] In some embodiments of this application, the pickling temperature of the hot-rolled steel coil is ≥75°C, and the acid concentration is ≥100g / L. Exemplarily, the acid concentration is 110g / L, 120g / L, 125g / L, 140g / L, 150g / L, 160g / L, 170g / L, or 180g / L. This application ensures the pickling effect and surface quality of the hot-rolled steel coil after galvanizing by controlling the pickling process.

[0038] In some embodiments of this application, the manufacturing method of galvanized duplex steel with different yield strength levels and 780MPa tensile strength levels further includes: performing finishing and tension leveling treatments on the galvanized steel sheet, wherein the finishing elongation of the finishing treatment on the galvanized steel sheet is 0.3% to 0.6%, the finishing rolling force is ≥200T, and the tension leveling elongation of the galvanized steel sheet is 0.05% to 0.15%.

[0039] In some embodiments of this application, manufacturing galvanized duplex steel with yield strength of 420MPa to 550MPa and tensile strength of 780MPa includes: hot rolling a continuously cast slab at 1220℃ to 1290℃ to obtain a hot-rolled steel coil; wherein the finishing rolling temperature in the hot rolling process is 850℃ to 930℃ and the coiling temperature is 560℃ to 640℃.

[0040] In some embodiments of this application, manufacturing galvanized duplex steel with yield strength of 500MPa to 650MPa and tensile strength of 780MPa includes: hot rolling a continuously cast slab at 1220℃ to 1290℃ to obtain a hot-rolled steel coil; wherein the finishing rolling temperature in the hot rolling process is 850℃ to 930℃ and the coiling temperature is 520℃ to 600℃.

[0041] The manufacturing method of this application achieves different yield strength levels in galvanized duplex steel products by employing different coiling processes, specifically by controlling the grain size and precipitates of the hot-rolled substrate (i.e., the continuously cast slab). This control begins with the properties of the hot-rolled substrate, resulting in different substrate strength indicators. In this application, the hot-rolling treatment and the coiling temperature after rolling play a crucial role in achieving different yield strength levels in galvanized duplex steel. By controlling different coiling temperatures and the grain size and precipitates of the hot-rolled continuously cast slab, different substrate strength indicators are achieved, thereby obtaining galvanized duplex steel products with different yield strength levels.

[0042] In the manufacturing method of this application, the single-stand cold rolling process plays an important role in achieving different yield strength levels. Single-stand rolling can ensure that the pickled steel coil has a suitable cold rolling reduction rate, thereby obtaining better performance and structure. It also ensures that the galvanized steel coil obtains high strength, stable performance and uniform structure after galvanizing and annealing, while solving the problem of difficulty in continuous rolling of ultra-high strength acid steel.

[0043] In the manufacturing method of this application, the annealing temperature before galvanizing plays an important role in achieving different yield strength levels. By controlling different annealing temperatures, the transformation amount of austenite, grain size, and transformation mode and amount of austenite during the heating process are controlled, thereby achieving different strength indicators for galvanized duplex steel.

[0044] The present application provides a simple and easy-to-control manufacturing process for preparing galvanized duplex steel with different yield strength levels and tensile strength levels of 780MPa using the above-described manufacturing method. It produces galvanized duplex steel with different yield strength levels of 420MPa and 500MPa and tensile strengths of 780MPa, with a thickness of 0.8mm to 2.0mm, which can meet the requirements of the automotive steel sheet industry.

[0045] The manufacturing method of galvanized duplex steel with different yield strengths and 780MPa tensile strength grades in this application mainly adopts continuous galvanizing annealing, and achieves the production of steel plates with different structures and properties through temperature control. The process is simple, highly operable, and low in cost.

[0046] In the manufacturing method of this application, the roles of the key chemical composition ratios and process steps are as follows:

[0047] 1) Key chemical composition ratios and their functions

[0048] C: Carbon is an indispensable and economical strengthening element in steel, affecting the strength and formability of steel plates. As the carbon content increases, the strength of the steel increases, but the formability decreases. Therefore, the carbon content should not be too high. In this application, the C content in the continuously cast slab should be controlled within the range of 0.06% to 0.09%.

[0049] Silicon (Si): Silicon is a strengthening element that improves the strength of materials; the higher the silicon content, the higher the material strength. However, high silicon content can easily lead to the formation of iron oxide scale that is difficult to pickle, and can also cause incomplete galvanizing after galvanizing, affecting the surface quality of the steel plate. Therefore, the Si content should be controlled between 0.10% and 0.25%.

[0050] Mn: Like silicon, manganese is a strengthening element for materials. Adding a small amount of manganese to steel can improve its strength. Simultaneously, adding a certain amount of manganese can combine with sulfur to form MnS, preventing hot brittleness and reducing the possibility of corrosion and perforation caused by MnS inclusions. However, adding too much manganese can easily lead to oxide formation during zinc plating, causing incomplete plating and also hindering stamping processes. Therefore, the Mn content should be controlled between 1.9% and 2.3%.

[0051] Cr: Chromium can significantly delay the transformation of pearlite and bainite, thereby allowing austenite to fully transform into martensite. Therefore, chromium is added in this invention. In this application, Cr should be controlled at 0.22%~0.30%.

[0052] P: Phosphorus can improve the strength of materials, but it can also cause cold brittleness. Therefore, the phosphorus content should be kept as low as possible, ideally ≤0.015%.

[0053] Sulfur (S) can increase material strength but reduce its plasticity and formability. It also causes hot brittleness, affecting the surface quality of steel plates. Furthermore, sulfur combines with manganese (Mn) in steel to form MnS inclusions, which can easily become the starting point for corrosion and perforation. Therefore, the sulfur content should be kept as low as possible, ideally controlled at ≤0.025%.

[0054] Als: Aluminum is added as a deoxidizer during steelmaking. When the content is less than 0.01%, the inclusions in the steel increase and the stamping performance deteriorates. At the same time, aluminum can refine the grains, fix the nitrogen in the steel, and increase the martensitic transformation temperature. Therefore, the Al content in this application should be controlled at 0.40%~0.70%.

[0055] Nitrogen (N) combines with aluminum to form AlN precipitates. Dispersed AlN particles hinder grain growth, refine grains, and significantly increase strength while reducing formability. Therefore, the N content should be controlled at ≤0.005%.

[0056] Niobium (Nb) can form carbon and nitrogen compounds with C and N, precipitating at the phase interface during the austenite-ferrite transformation, thus preventing ferrite grain growth and refining the grain size. When Nb is added to a certain value, precipitation strengthening can occur. In this application, the Nb content should be controlled between 0.01% and 0.025%.

[0057] Mo: Molybdenum can dissolve in ferrite, austenite, and carbides in steel, improving its hardenability and reducing the austenite phase region. Molybdenum strengthens ferrite through solid solution and also improves the stability of carbides, thus increasing the strength of the steel. Furthermore, the addition of molybdenum significantly improves the stability of the metastable microstructure of the steel, ensuring the acquisition of a two-phase microstructure. In this application, the Mo content should be controlled between 0.15% and 0.20%.

[0058] 2) Key process steps and their functions

[0059] ① Finishing rolling temperature: The finishing rolling temperature in hot rolling is set at an austenitic temperature higher than Ar3. If the temperature is too high, the iron oxide scale will be severe and difficult to pickle, affecting the surface quality of the steel plate; if the temperature is too low, entering the two-zone rolling will result in mixed crystals in the material, producing snowflake defects during stamping, affecting the appearance of enamel products. Therefore, the finishing rolling temperature in hot rolling is generally controlled between 850℃ and 930℃.

[0060] ② Coiling Temperature: Higher coiling temperatures promote grain growth, reduce strength, and improve formability. However, excessively high coiling temperatures can lead to severe iron oxide scale on the steel plate surface, making pickling difficult and affecting the quality of the steel plate. Therefore, the coiling temperature control range is: 560℃~640℃ for yield strength of 420MPa and 520℃~600℃ for yield strength of 500MPa.

[0061] ③ Pickling process: Hot-rolled steel coils inevitably generate iron oxide scale. Under the condition that the iron oxide scale is controllable, an unreasonable pickling process will lead to incomplete pickling of the surface iron oxide scale, resulting in oxide intrusion and over-pickling. Therefore, the pickling process should be controlled as follows: pickling temperature ≥75℃, acid concentration ≥100g / L.

[0062] ③ Cold rolling deformation: The pickled steel coil is cold rolled and coiled to obtain a cold-hardened coil. A larger cold rolling deformation can reduce the recrystallization temperature and phase transformation temperature, obtain higher strength, and ensure the uniformity of strength and structure; however, excessive deformation will increase the rolling difficulty. Therefore, the cold rolling deformation should be selected according to the actual situation of a single stand, and the total cold rolling reduction rate is between 55% and 67%.

[0063] ④ Galvanizing annealing temperature: A suitable heating zone temperature can ensure the production of a suitable amount of austenite in the annealing furnace. Too high or too low a temperature will result in too much or too little austenite. After cooling, a suitable martensite and ferrite dual-phase structure cannot be obtained. Therefore, the annealing temperature for a yield strength of 420MPa is controlled between 755℃ and 770℃, and the annealing temperature for a yield strength of 500MPa is controlled between 775℃ and 790℃.

[0064] ⑤ Smoothing elongation: The purpose of smoothing is to eliminate the yield plateau of the steel plate, avoid tensile strain mark defects during the stamping process, improve the plate shape, and enhance the surface quality of galvanized steel. Galvanized duplex steel products are characterized by the absence of a yield plateau; the main purpose of smoothing galvanized duplex steel is to improve the plate shape and surface quality. Therefore, a smoothing elongation range of 0.3% to 0.6% is more suitable.

[0065] ⑥ Leveling elongation: The purpose of leveling is to adjust and optimize the sheet shape, avoiding edge and center waviness in the material. If the leveling elongation is too low, the sheet shape will be poor; if the leveling elongation is too high, leveling marks are likely to appear. Therefore, a leveling elongation range of 0.05% to 0.15% is more suitable.

[0066] The technical solution of this application will be further described in detail below with reference to specific embodiments.

[0067] Examples 1-10

[0068] According to the manufacturing methods of galvanized duplex steel with different yield strength grades and 780MPa tensile strength grades in this application, and in accordance with the steel smelting process, the following continuously cast slabs conforming to the composition specifications of continuously cast slabs are provided as shown in Table 1:

[0069] Table 1. Chemical composition of continuously cast slabs from Examples 1-10 and Comparative Examples 1-3

[0070]

[0071] The continuously cast slabs provided in Examples 1-10 above are heated in a heating furnace to achieve a furnace exit temperature of 1220℃~1290℃. After being rough-rolled for 7 passes, the intermediate slab thickness is 40mm. The intermediate slab is then finished by finishing rolling at a temperature of 850℃~900℃ to obtain hot-rolled steel coils. The coiling temperature of the hot-rolled steel coils in Examples 1-5 is 560℃~640℃, and the coiling temperature of the hot-rolled steel coils in Examples 6-10 is 520℃~600℃.

[0072] After hot-rolled steel coils are straightened to remove zero phosphorus, they are pickled. The pickling temperature is ≥75℃ and the HCl concentration in the pickling bath is ≥100g / L to obtain pickled steel coils.

[0073] Pickled steel coils are cold rolled using a single-stand, six-pass rolling mill to obtain cold-rolled coils with a thickness of 0.8 mm to 2.0 mm; the total reduction rate of the cold rolling process is 55% to 67%.

[0074] After continuous annealing, the cold-rolled steel coils were galvanized. The annealing temperature of the galvanized duplex steel in Examples 1-5 was controlled at 755℃~770℃, and the annealing temperature of the galvanized duplex steel in Examples 6-10 was controlled at 775℃~790℃. The finishing rolling force was ≥200T, and the tensile elongation was 0.05%~0.15%. Finally, the coils were taken from the inlet and outlet to obtain hot-dip galvanized steel coils with a thickness of 0.8mm~2.0mm. The specific process parameters for preparing the galvanized duplex steel in Examples 1-10 and Comparative Examples 1-3 are shown in Table 2.

[0075] Table 2. Rolling process parameters for galvanized duplex steels in Examples 1-10 and Comparative Examples 1-3.

[0076]

[0077] The galvanized duplex steels of Examples 1-10 and Comparative Examples 1-3 were subjected to performance tests according to GB / T 228-2010, and the mechanical property test results of Examples 1-10 and Comparative Examples 1-3 are recorded in Table 3 below:

[0078] Table 3. Mechanical property test results of galvanized duplex steels in Examples 1-10 and Comparative Examples 1-3

[0079]

[0080] As can be seen from Tables 2 and 3, compared with the galvanized duplex steel of Comparative Examples 1-3, the galvanized duplex steel produced does not meet the limits of the component content or process parameters of the manufacturing method of this application. The galvanized duplex steel produced either has a qualified elongation after fracture but not qualified yield strength and tensile strength, or qualified yield strength and tensile strength but not qualified elongation after fracture, or qualified tensile strength and elongation after fracture but not qualified yield strength.

[0081] According to the manufacturing methods of galvanized duplex steel with different yield strength levels and a tensile strength level of 780 MPa in this application, Examples 1-5 prepared galvanized duplex steel with a yield strength of 440 MPa to 475 MPa and a tensile strength of 780 MPa at a coiling temperature of 580℃ to 618℃, with an elongation after fracture of over 19%. Examples 6-10 prepared galvanized duplex steel with a yield strength of 529 MPa to 569 MPa and a tensile strength of 780 MPa at a coiling temperature of 542℃ to 577℃, with an elongation after fracture of over 15%, meeting the requirements for galvanized duplex steel for automotive applications. As shown in Figure 2, the microstructure of the galvanized duplex steel in Example 3 is ferrite + martensite, where M represents martensite and F represents ferrite. As shown in Figure 3, the microstructure of the galvanized duplex steel in Example 7 is ferrite + martensite, where M represents martensite and F represents ferrite. The grain sizes of martensite and ferrite in Figures 2 and 3 show significant differences. The metallographic diagrams of Examples 3 and 7 demonstrate that the manufacturing method of this application, through different process controls, can ultimately obtain metallographic structures with different grain sizes, thereby enabling the production of galvanized duplex steels with different yield strengths and tensile strengths of 780 MPa. The ferrite and martensite structures in Figures 2 and 3 are relatively uniformly distributed, but the martensite density in Figure 3 is higher and the structure is finer, resulting in correspondingly higher yield strength and tensile strength.

[0082] The above description is merely a specific implementation of this application. Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working process described above can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here. It should be understood that the protection scope of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the protection scope of this application.

Claims

1. A method for manufacturing galvanized duplex steel with different yield strength grades and a tensile strength grade of 780 MPa, characterized in that, include: A continuously cast slab is provided; the continuously cast slab comprises the following components by mass percentage: C: 0.06%~0.09%, Si: 0.10%~0.25%, Mn: 1.9%~2.3%, Als: 0.40%~0.70%, Nb: 0.01%~0.025%, Cr: 0.22%~0.30%, Mo: 0.15%~0.20%, P≤0.015%, S≤0.025%, N≤0.005%, with the remainder being Fe and unavoidable impurities; The continuously cast slab is hot-rolled at 1220℃~1290℃ to obtain a hot-rolled steel coil; wherein the finishing rolling temperature in the hot rolling process is 850℃~930℃ and the coiling temperature is 520℃~640℃. The hot-rolled steel coil is pickled to obtain a pickled steel coil; The pickled steel coil is cold-rolled to obtain a cold-hardened coil; wherein the total reduction rate of the cold rolling is 55% to 67%. The cold-rolled coil is annealed and galvanized to obtain a galvanized steel sheet; wherein the annealing temperature is 755℃~770℃ or 775℃~790℃.

2. The manufacturing method according to claim 1, characterized in that, Also includes: The galvanized steel sheet is subjected to finishing and tension leveling treatments; wherein the finishing elongation of the finishing treatment is 0.3% to 0.6%, the finishing rolling force is ≥200T, and the tension leveling elongation of the finishing treatment is 0.05% to 0.15%.

3. The manufacturing method according to claim 1, characterized in that, The hot rolling process of the continuously cast slab at 1220℃~1290℃ includes: The continuously cast slab is heated to 1220℃~1290℃; The continuously cast slab heated to 1220℃~1290℃ is subjected to rough rolling and finish rolling processes in sequence to obtain hot-rolled steel. The hot-rolled steel is coiled to obtain a hot-rolled steel coil.

4. The manufacturing method according to claim 1, characterized in that, At least one of the following requirements must be met: The thickness of the continuously cast slab is 220mm to 260mm; The thickness of the hot-rolled steel coil is 2.4 mm to 4.5 mm.

5. The manufacturing method according to claim 1, characterized in that, The continuous casting slab is heated at 1220℃~1290℃ for 150 minutes to 400 minutes.

6. The manufacturing method according to claim 1, characterized in that, The pickling temperature of the hot-rolled steel coil is ≥75℃, and the acid concentration is ≥100g / L.

7. The manufacturing method according to claim 1 or 2, characterized in that, The manufacture of galvanized duplex steel with yield strengths of 420MPa to 550MPa and tensile strengths of 780MPa includes: The continuously cast slab is hot-rolled at 1220℃~1290℃ to obtain hot-rolled steel coils; the finishing rolling temperature in the hot rolling process is 850℃~930℃, and the coiling temperature is 560℃~640℃.

8. The manufacturing method according to claim 1 or 2, characterized in that, The manufacture of galvanized duplex steel with yield strengths of 500MPa to 650MPa and tensile strengths of 780MPa includes: The continuously cast slab is hot-rolled at 1220℃~1290℃ to obtain hot-rolled steel coils; the finishing rolling temperature in the hot rolling process is 850℃~930℃, and the coiling temperature is 520℃~600℃.

9. A galvanized duplex steel, characterized in that, The galvanized duplex steel is prepared by the manufacturing method of different yield strength levels and 780MPa tensile strength levels according to any one of claims 1-7, wherein the galvanized duplex steel has a yield strength of 420MPa to 550MPa, a tensile strength of ≥780MPa, and an elongation of ≥17%.

10. A galvanized duplex steel, characterized in that, The galvanized duplex steel is prepared by the manufacturing method of different yield strength levels and 780MPa tensile strength levels according to any one of claims 1-6 and 8, wherein the galvanized duplex steel has a yield strength of 500MPa to 650MPa, a tensile strength ≥780MPa, and an elongation ≥14%.

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