Method for producing low-cost high-strength galvanized sheet based on thin-strip cast rolling

Abstract

The invention belongs to the technical field of steel production, and relates to a method for producing a low-cost high-strength galvanized sheet based on thin-strip cast rolling. After molten steel is subjected to thin-strip continuous casting to obtain a casting strip, the casting strip is rolled into a thin strip in a single pass mode, the thin strip is coiled after being cooled through aerial fog, then hot-dip galvanizing is conducted on a continuous hot-dip galvanizing production line after acid pickling is conducted, the annealing temperature of a galvanizing section is 640-720 DEG C, and then the thin strip is placed at the room temperature for more than 35 days after leveling is conducted. According to the method, supersaturated solid solution of C and N atoms in the thin strip casting and rolling sub-rapid solidification process is fully utilized, the high solid solution state is maintained by combining hot rolling and galvanizing section short-time and low-temperature design, finally, strength jump is driven by means of natural aging after production, and the high-strength galvanized plate with good stamping performance is obtained under a simple C-Si-Mn component system. The problems that the high-strength galvanized plate forming capacity of a traditional high-carbon route is insufficient, and the alloy cost of a low-carbon microalloy route is high are solved, and meanwhile carbon emission in the product production process is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of steel production and relates to a method for producing low-cost high-strength galvanized sheets based on thin strip casting and rolling. Background Art

[0002] Galvanized sheet is widely used in construction, home appliances, automobiles and other fields due to its good corrosion resistance. The composition system of galvanized sheet varies significantly due to the application field and processing requirements. Construction panels generally use a higher carbon content (such as 0.15-0.18%) to obtain ferrite + pearlite structure, relying on solid solution strengthening and pearlite structure strengthening to reduce costs. However, this type of structure has poor plasticity due to the presence of pearlite, making it difficult to meet complex stamping requirements. It is only suitable for bending or simple stamping and other processing.

[0003] Due to the complex deformation during processing, household appliance and automotive sheets require a single-phase ferrite structure to ensure good plasticity and formability. The carbon content is generally less than 0.07%, but achieving high strength at low carbon content requires expensive alloy additions. For example, the 340MPa grade alloyed hot-dip galvanized sheet (CN117165845A) developed by Anshan Iron and Steel adopts a microalloying technology route. After adding Nb (0.04-0.07%) to the composition, the alloy cost per ton of steel increases by about 200 yuan. The 400MPa grade galvanized sheet (CN202410631418.6) developed by Hegang adopts a rare earth technology route, introducing Ce (0.008-0.015%) and B (0.0015-0.003%) to purify the grain boundaries, but the rare earth yield fluctuates greatly and the process window is narrow. The above technical routes all rely on alloying elements other than C-Si-Mn to achieve high strength at low carbon content, which leads to a significant increase in alloy costs and increases the complexity of process control, deviating from the industry development trend of low cost and short process.

[0004] In this context, natural aging (solute atom diffusion strengthening at room temperature) could be a cost-free strengthening method, but its potential is severely suppressed under traditional processes. The traditional galvanized sheet production process is: molten steel smelting → thick slab continuous casting → rough rolling → multi-stand finishing rolling → laminar cooling → coiling → pickling → cold rolling → annealing → galvanizing → flattening. This process is lengthy and involves multiple manufacturing steps. Due to the thick slabs, the conventional hot rolling process often uses slow cooling and high-temperature coiling. Furthermore, to achieve a fully recrystallized structure after cold rolling, the annealing temperature during the galvanizing stage is also high. These process characteristics lead to the precipitation of large amounts of carbides, resulting in a dissolved carbon content of less than 20 ppm in the finished galvanized product. Consequently, the natural aging strengthening effect is minimal (yield strength increases by approximately 20-30 MPa, while tensile strength remains essentially unchanged or increases by 10 MPa). Furthermore, residual free carbon atoms pose risks such as Lüders banding. The above problems force existing technologies to focus on eliminating natural aging, generally through ultra-low carbon design (C ≤ 0.003%), or by using microalloys to fix carbon and nitrogen atoms, or even using surface decarburization processes. Even if attempts are made to develop and utilize the aging effect (such as Nippon Steel CN103975082A), Cu and Ni must still be added and combined with pre-aging heat treatment. In other words, under current technical conditions, it is impossible to achieve a significant increase in strength by using natural aging in a C-Si-Mn based composition system.

[0005] Strip casting is a short process and near-net shape processing technology that integrates sub-rapid solidification and rolling deformation. Its sub-rapid solidification characteristics (cooling rate>10 3 ℃ / s) provides a new path to overcome solubility limitations. This process can achieve a solid solubility of carbon and nitrogen atoms exceeding 100 ppm after solidification, far exceeding traditional continuous casting processes. Furthermore, the thin strip casting process eliminates the heating furnace and multiple roughing steps required for traditional hot rolling. The multi-stand finishing rolling process is streamlined to a single stand, allowing the production of 0.6mm-1.8mm hot-rolled thin strip directly from molten steel within 30 seconds. Consequently, cold rolling can even be omitted when producing galvanized products within this specification.

[0006] The process for producing galvanized sheet from cast and rolled strip coils is as follows: molten steel smelting → continuous strip casting → single-stand finishing rolling → laminar cooling → coiling → pickling → annealing → galvanizing → flattening. Compared to the traditional process, this process is significantly streamlined. Under these conditions, the time from molten steel solidification to hot-rolled coiling is short (30 seconds for strip casting, compared to several hours for traditional continuous casting and rolling). Furthermore, since cold rolling is not required, the annealing temperature during the galvanizing stage can be lowered. The short, low-temperature nature of the hot rolling and galvanizing annealing stages helps maintain a high solid solution state of carbon and nitrogen atoms. Therefore, the driving force and effectiveness of the subsequent natural aging stage are greatly enhanced during the storage period after the galvanized product is produced. During storage, the carbon and nitrogen atoms in high solid solution diffuse around dislocations, creating a significant pinning effect on them, significantly increasing the resistance to further dislocation movement and thus significantly improving material strength.

[0007] However, the industry has yet to explore the strengthening value of high-state atoms during natural aging under thin strip casting, let alone their integration with the C-Si-Mn base composition. Furthermore, the development of high-strength galvanized sheet continues to rely on the addition of precious alloys. Given the current state of these technologies, the industry urgently needs an innovative solution that can both avoid high alloy costs and unlock the potential of natural aging. Summary of the Invention

[0008] In view of the above problems, the present invention provides a method for producing low-cost, high-strength galvanized sheet based on thin strip casting and rolling. Under the basic composition of only C-Si-Mn, the supersaturated solid solution of C and N atoms is achieved by thin strip casting and rolling through sub-rapid solidification. The high solid solution state is maintained through the extremely short process of thin strip casting and rolling and the short-time, low-temperature annealing in the galvanizing section. Finally, the strength is driven to jump by natural aging after production.

[0009] To achieve the above object, the technical solution provided by the present invention is:

[0010] According to a first aspect of the present invention, a method for producing low-cost high-strength galvanized sheet based on thin strip casting is provided, the method comprising the following steps:

[0011] (1) smelting to obtain molten steel, wherein the molten steel is prepared according to the following chemical composition and mass fraction:

[0012] C: 0.02-0.04%, Mn: 0.3-0.8%, Si: 0.05-0.20%, P: ≤0.02% and greater than 0, S: ≤0.005% and greater than 0, N: 0.002%-0.006%, the balance being Fe and impurity elements;

[0013] (2) performing thin strip continuous casting on the molten steel obtained in step (1) to obtain a cast strip with a thickness of 1.4 to 2.5 mm, wherein the protective atmosphere for casting is nitrogen;

[0014] (3) subjecting the cast strip obtained in step (2) to a single-pass hot rolling process to obtain a thin strip having a thickness of 0.6 to 2.0 mm, with the hot rolling outlet temperature being 780 to 930° C.;

[0015] (4) cooling the thin strip obtained in step (3) to 550-650° C. through an air mist cooling system, and coiling it to obtain a hot-rolled coil;

[0016] (5) air-cooling the hot-rolled coil obtained in step (4) to room temperature, and then pickling it. After pickling, annealing and hot-dip galvanizing are performed on a continuous hot-dip galvanizing production line to obtain a galvanized coil;

[0017] (6) leveling the galvanized coil obtained in step (5) through a leveler to obtain a finished galvanized coil, wherein the leveling elongation of the leveler is 0.8 to 1.5%;

[0018] (7) placing the galvanized coil obtained in step (6) at room temperature for more than 35 days.

[0019] According to the method for producing low-cost high-strength galvanized sheet based on thin strip casting and rolling of the present invention, preferably, in step (1), no Nb, V, Ti, Cr, or Mo alloying elements are added to the chemical composition of the molten steel.

[0020] According to the method for producing low-cost high-strength galvanized sheet based on thin strip casting and rolling of the present invention, preferably, in step (1), EAF electric furnace steelmaking, VD vacuum decarburization and deoxidation, and LF refining are adopted to obtain molten steel with qualified composition.

[0021] According to the method for producing low-cost high-strength galvanized sheet based on thin strip casting of the present invention, preferably, in step (3), the hot rolling reduction is 20% to 70%.

[0022] According to the method for producing low-cost high-strength galvanized sheet based on thin strip casting and rolling of the present invention, preferably, in step (3), the hot rolling outlet temperature is 850-930°C.

[0023] According to the method for producing low-cost high-strength galvanized sheet based on thin strip casting of the present invention, preferably, in step (3), the hot rolling outlet temperature is 850-870°C.

[0024] According to the method for producing low-cost high-strength galvanized sheet based on thin strip casting and rolling of the present invention, preferably, in step (5), the annealing temperature of the hot-dip galvanizing production line is 640-720°C.

[0025] According to the method for producing low-cost high-strength galvanized sheet based on thin strip casting and rolling of the present invention, preferably, in step (5), the annealing temperature of the hot-dip galvanizing production line is 700-720°C.

[0026] According to a second aspect of the present invention, there is provided a galvanized sheet, which is produced by the method having the above characteristics.

[0027] According to the galvanized sheet of the present invention, preferably, the metallographic structure of the galvanized sheet is a single-phase ferrite structure and does not contain pearlite structure.

[0028] According to the galvanized sheet of the present invention, preferably, the galvanized sheet has a yield strength of 220 to 280 MPa and a tensile strength of 340 to 400 MPa when it is just off the production line.

[0029] According to the galvanized sheet of the present invention, preferably, after being placed at room temperature for more than 35 days, the yield strength of the galvanized sheet increases by more than 60 MPa, and the tensile strength increases by more than 40 MPa compared to when it is just off the production line.

[0030] Beneficial technical effects

[0031] Compared with the prior art, the technical concept and corresponding technical solutions of the present invention can at least achieve the following beneficial technical effects:

[0032] (1) The low-cost, high-strength galvanized sheet based on thin strip casting of the present invention does not add any additional alloying elements such as Nb, V, Ti, Cr, and Mo except for C, Si, and Mn. Compared with the traditional low-carbon + micro-alloying technology solution, the alloy cost is greatly reduced.

[0033] (2) The metallographic structure of the low-cost, high-strength galvanized sheet based on thin strip casting of the present invention is single-phase ferrite. Compared with the traditional C-Si-Mn alloy system material with a higher carbon content (the structure is ferrite + pearlite), the low-cost, high-strength galvanized sheet of the present invention does not contain pearlite, has better forming ability, and can meet more complex stamping requirements.

[0034] (3) The low-cost, high-strength galvanized sheet based on thin strip casting of the present invention utilizes the sub-rapid solidification of thin strip casting to achieve supersaturated solid solution of C and N atoms, maintains a high solid solution state through an extremely short thin strip casting process and a short low-temperature annealing in the galvanizing section, and achieves significant strength improvement after natural aging, with a yield strength increase of more than 80 MPa and a tensile strength increase of more than 50 MPa.

[0035] (4) The low-cost, high-strength galvanized sheet based on thin strip casting of the present invention uses electric furnace smelting for molten steel, which can adopt a higher scrap steel ratio, and the carbon emissions of the production process are greatly reduced.

[0036] (5) The low-cost, high-strength galvanized sheet based on thin strip casting of the present invention adopts a short-process production process of thin strip casting as the hot-rolled raw material, while eliminating the cold rolling process. The production process is reduced, and the production cost and labor cost are greatly reduced.

[0037] (6) This invention opens up a new direction for the development of solution aging resources in the thin strip casting and rolling process, providing a disruptive path for the production of low-cost, high-performance galvanized sheets. At the same time, based on the short-process steelmaking + short-process casting and rolling process of electric furnace + thin strip casting and rolling, the carbon emissions of the product production process are greatly reduced, providing the industry with a new paradigm of low-cost, low-energy consumption, high-performance, and green steel manufacturing, which has great application prospects in the context of dual carbon. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings of the embodiments. Obviously, the drawings described below only relate to some embodiments of the present invention, and are not intended to limit the present invention.

[0039] Figure 1This is the metallographic structure (single-phase ferrite) of the low-cost, high-strength galvanized sheet based on thin strip casting in Example 1 of the present invention.

[0040] Figure 2 The metallographic structure (ferrite + pearlite) of the galvanized sheet with higher carbon content under the conventional process in Comparative Example 2 is shown in FIG. DETAILED DESCRIPTION

[0041] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] Unless otherwise defined, technical or scientific terms used in the present invention shall have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs.

[0043] Example 1

[0044] 1) Smelting: EAF electric furnace steelmaking, VD vacuum decarburization and deoxidation, LF refining are adopted to obtain molten steel with the following composition by weight percentage: C: 0.033, Mn: 0.46, Si: 0.011, P: 0.012, S: 0.002, N: 0.004, and the balance is Fe and impurity elements.

[0045] 2) Thin strip continuous casting: The molten steel is cast and rolled using a twin-roll thin strip casting and rolling equipment. The molten steel is continuously cast in a thin strip under nitrogen gas protection to obtain a cast strip with a thickness of 2.05 mm.

[0046] 3) Hot rolling: The cast strip is then hot rolled into a 1.4 mm thin strip with an outlet temperature of 870°C.

[0047] 4) Cooling and coiling: The hot-rolled thin strip is cooled to 640°C by a mist cooling system, coiled to obtain a hot-rolled coil, and then air-cooled to room temperature.

[0048] 5) Pickling and galvanizing: After pickling, the hot rolled coil is annealed and galvanized in a continuous galvanizing line, where the annealing temperature is 720℃.

[0049] 6) Leveling: The galvanized strip is leveled by a leveler, and the leveling elongation is 1.3%.

[0050] 7) Place at room temperature: Place the finished galvanized coil at room temperature for 40 days after it comes off the production line.

[0051] The performance of the material after 40 days of galvanizing is shown in Table 1. The metallographic structure of the material is shown in Figure 1.

[0052] Table 1 Mechanical properties of low-cost high-strength galvanized sheet based on thin strip casting in Example 1

[0053]

[0054] Example 2

[0055] 1) Smelting: EAF electric furnace steelmaking, VD vacuum decarburization and deoxidation, LF refining are adopted to obtain molten steel with the following composition by weight percentage: C: 0.027, Mn: 0.38, Si: 0.01, P: 0.013, S: 0.003, N: 0.005, and the balance is Fe and impurity elements.

[0056] 2) Thin strip continuous casting: The molten steel is cast and rolled using a twin-roll thin strip casting and rolling equipment. The molten steel is continuously cast in a thin strip under nitrogen gas protection to obtain a cast strip with a thickness of 1.85 mm.

[0057] 3) Hot rolling: The cast strip is then hot rolled into a 0.8 mm thin strip with an outlet temperature of 850°C.

[0058] 4) Cooling and coiling: The hot-rolled thin strip is cooled to 620°C by a mist cooling system, coiled to obtain a hot-rolled coil, and then air-cooled to room temperature.

[0059] 5) Pickling and galvanizing: After pickling, the hot rolled coil is annealed and galvanized in a continuous galvanizing line, where the annealing temperature is 700℃.

[0060] 6) Leveling: The galvanized strip is leveled by a leveler, and the leveling elongation is 1.1%.

[0061] 7) Place at room temperature: Place the finished galvanized coil at room temperature for 50 days after it comes off the production line.

[0062] The performance of the galvanized steel after 50 days at room temperature is shown in Table 2.

[0063] Table 2 Mechanical properties of low-cost high-strength galvanized sheet based on thin strip casting in Example 2

[0064]

[0065] Comparative Example 1

[0066] The production process is: traditional slab continuous casting + conventional hot rolling + pickling + cold rolling + galvanizing + leveling + room temperature storage. The composition is a micro-alloyed high-strength route of C-Si-Mn-Nb, and the alloy cost is relatively high.

[0067] 1) Smelting: Molten steel is subjected to hot metal pre-desulfurization, converter top and bottom combined blowing, RH vacuum cycle deoxidation, calcium treatment, and LF refining to obtain a qualified molten steel composition. The following weight percentages are: C: 0.057, Mn: 0.38, Si: 0.016, Nb: 0.053, T: 0.005, P: 0.032, S: 0.004, N: 0.0023, with the remainder being Fe and impurities.

[0068] 2) Continuous Casting and Rolling: Continuously cast into 220mm thick ingots, which are then heated to 1250°C and held for at least 2 hours for soaking treatment. Rough and finish rolling then produces 3mm thick strips, with a final rolling temperature of 910°C and a coiling temperature of 480°C.

[0069] 3) Pickling and cold rolling: After pickling, the hot rolled coil is cold rolled to 0.8mm.

[0070] 4) Galvanizing and leveling: The cold rolled coil is annealed and galvanized in a continuous galvanizing line at an annealing temperature of 830°C. The galvanized strip is leveled in a leveler with a leveling elongation of 0.8%.

[0071] 5) Place at room temperature: Place the finished galvanized coil at room temperature for 40 days after it comes off the production line.

[0072] The performance of the galvanized steel after 40 days at room temperature is shown in Table 3.

[0073] Table 3 Mechanical properties of microalloyed high strength galvanized sheet under conventional process in comparative example 1

[0074]

[0075] Comparative Example 2

[0076] The production process is: traditional slab continuous casting + conventional hot rolling + pickling + cold rolling + galvanizing + flattening + room temperature storage. The composition is C-Si-Mn, but the C content is relatively high (0.017%), the structure is ferrite + pearlite, and the stamping forming ability is poor.

[0077] 1) Smelting: Molten steel is smelted in a converter and refined to obtain qualified molten steel with the following weight percentages: C: 0.17, Mn: 0.42, Si: 0.03, P: 0.014, S: 0.005, with the remainder being Fe and impurity elements.

[0078] 2) Continuous Casting and Rolling: Continuously cast into 220mm thick ingots, which are then heated to 1210°C and held for at least 2 hours for soaking treatment. Rough and finish rolling then produces 3.5mm thick strip steel, with a final rolling temperature of 890°C and a coiling temperature of 570°C.

[0079] 3) Pickling and cold rolling: After pickling, the hot rolled coil is cold rolled to 1.2mm.

[0080] 4) Galvanizing and leveling: The cold rolled coil is annealed and galvanized in a continuous galvanizing line at an annealing temperature of 785°C. The galvanized strip is leveled in a leveler with a leveling elongation of 1.1%.

[0081] 5) Place at room temperature: Place the finished galvanized coil at room temperature for 40 days after it comes off the production line.

[0082] The performance of the material after 40 days of galvanizing is shown in Table 4. The metallographic structure of the material is shown in Figure 2 .

[0083] Table 4 Mechanical properties of 0.17% C content galvanized sheet under conventional process in comparative example 2

[0084]

[0085] The above description is only a specific embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, they can make several improvements and modifications without departing from the principles of the present invention. These improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing low-cost high-strength galvanized sheet based on thin strip casting, characterized in that: The method comprises the following steps: (1) smelting to obtain molten steel, wherein the molten steel is prepared according to the following chemical composition and mass fraction: C: 0.02-0.04%, Mn: 0.3-0.8%, Si: 0.05-0.20%, P: ≤0.02% and greater than 0, S: ≤0.005% and greater than 0, N: 0.002%-0.006%, the balance being Fe and impurity elements; (2) performing thin strip continuous casting on the molten steel obtained in step (1) to obtain a cast strip with a thickness of 1.4 to 2.5 mm, wherein the protective atmosphere for casting is nitrogen; (3) subjecting the cast strip obtained in step (2) to a single-pass hot rolling process to obtain a thin strip having a thickness of 0.6 to 2.0 mm, with the hot rolling outlet temperature being 780 to 930° C.; (4) cooling the thin strip obtained in step (3) to 550-650° C. through an air mist cooling system, and coiling it to obtain a hot-rolled coil; (5) air-cooling the hot-rolled coil obtained in step (4) to room temperature, and then pickling it. After pickling, annealing and hot-dip galvanizing are performed on a continuous hot-dip galvanizing production line to obtain a galvanized coil; (6) leveling the galvanized coil obtained in step (5) through a leveler to obtain a finished galvanized coil, wherein the leveling elongation of the leveler is 0.8 to 1.5%; (7) placing the galvanized coil obtained in step (6) at room temperature for more than 35 days.

2. The method for producing low-cost high-strength galvanized sheet based on thin strip casting according to claim 1, characterized in that: In step (1), no alloying elements Nb, V, Ti, Cr, or Mo are added to the chemical composition of the molten steel.

3. The method for producing low-cost high-strength galvanized sheet based on thin strip casting according to claim 1, characterized in that: In step (1), EAF electric furnace steelmaking, VD vacuum decarburization and deoxidation, and LF refining are adopted to obtain molten steel with qualified composition.

4. The method for producing low-cost high-strength galvanized sheet based on thin strip casting according to claim 1, characterized in that: In step (3), the hot rolling reduction is 20% to 70%.

5. The method for producing low-cost high-strength galvanized sheet based on thin strip casting according to claim 1, characterized in that: In step (3), the hot rolling outlet temperature is 850-930°C.

6. The method for producing low-cost high-strength galvanized sheet based on thin strip casting according to claim 5, characterized in that: In step (3), the hot rolling outlet temperature is 850-870°C.

7. The method for producing low-cost high-strength galvanized sheet based on thin strip casting according to claim 1, characterized in that: In step (5), the annealing temperature of the hot-dip galvanizing production line is 640-720°C.

8. The method for producing low-cost high-strength galvanized sheet based on thin strip casting according to claim 7, characterized in that: In step (5), the annealing temperature of the hot-dip galvanizing production line is 700-720°C.

9. A galvanized sheet, characterized in that: The galvanized sheet is produced by the method for producing low-cost high-strength galvanized sheet by thin strip casting according to any one of claims 1 to 8.

10. The galvanized sheet according to claim 9, characterized in that: The metallographic structure of the galvanized sheet is a single-phase ferrite structure and does not contain pearlite structure.

11. The galvanized sheet according to claim 9, characterized in that: When the galvanized sheet just comes off the production line: The yield strength is between 220 and 280 MPa, and the tensile strength is between 340 and 400 MPa.

12. The galvanized sheet according to claim 11, characterized in that: After the galvanized sheet was placed at room temperature for more than 35 days, compared with the time it just came off the production line: The yield strength increases by more than 60MPa and the tensile strength increases by more than 40MPa.

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