A 780MPa grade hot-dip galvanized DH steel and its manufacturing method

Abstract

This invention discloses a 780MPa grade hot-dip galvanized DH steel and its manufacturing method. By optimizing the alloy composition of DH780 steel and reducing the content of Si and Mn elements, the formation of external oxides is reduced, thereby lowering the risk of incomplete coating defects. In addition, by increasing the dew point during the annealing process, the oxygen partial pressure in the annealing atmosphere is increased, promoting the internal oxidation of Si and Mn elements on the surface of DH780 steel, thus effectively reducing the occurrence of incomplete coating defects and significantly improving the surface quality of the steel plate. This method not only reduces production costs but also improves the weldability and processability of the product, effectively improves the integrity and uniformity of the coating, enhances the corrosion resistance and service life of the product, and broadens the application range of DH780 hot-dip galvanized steel plates.

Description

Technical Field

[0001] This invention belongs to the field of high-strength steel technology, specifically relating to a 780MPa grade hot-dip galvanized DH steel and its manufacturing method. Background Technology

[0002] In the production process of hot-dip galvanized steel sheets, the surface quality of DH780 steel, a high-strength and high-toughness steel, is a key indicator for evaluating product quality. However, incomplete coating defects have always been a common problem affecting the surface quality of DH780 hot-dip galvanized steel sheets; incomplete coating not only damages the product's appearance but also reduces its corrosion resistance and service life, increasing subsequent maintenance and replacement costs.

[0003] In existing technologies, reducing the Si and Mn content to decrease external oxidation and plating defects has limited effectiveness. This is because Si and Mn play crucial roles in improving the strength and other properties of steel, and excessively reducing their content may impair the overall performance of the steel. For example, the patent application CN202311146689.4, entitled "A Low-Cost, High-Surface-Quality Hot-Dip Galvanized High-Strength Steel of 590MPa Grade and Its Production Method," describes a pre-oxidation treatment that improves the wettability and adhesion of the steel surface and enhances the uniformity and integrity of the coating. However, relying solely on pre-oxidation treatment cannot completely solve the plating defects caused by external oxidation of Si and Mn elements. Summary of the Invention

[0004] To overcome the shortcomings of the existing technology, the present invention aims to reduce the risk of incomplete plating defects by optimizing the alloy composition of DH780 steel, reducing the content of Si and Mn elements, and decreasing the formation of external oxides. Furthermore, by increasing the dew point during annealing, the oxygen partial pressure in the annealing atmosphere is increased, promoting internal oxidation of Si and Mn elements on the surface of DH780 steel, thereby effectively reducing the occurrence of incomplete plating defects and significantly improving the surface quality of the steel sheet. This method not only reduces production costs but also improves the weldability and processability of the product, effectively enhances the integrity and uniformity of the coating, improves the corrosion resistance and service life of the product, and broadens the application range of DH780 hot-dip galvanized steel sheets.

[0005] To achieve the above-mentioned objective, this invention provides a 780MPa grade hot-dip galvanized DH steel, wherein the chemical composition of the hot-dip galvanized DH steel is as follows: C: 0.15%–0.18%, Mn: 1.6%–2.0%, Si: 0.2%–0.6%, Cr: 0.10%–0.15%, Al: 0.4%–1.0%, Nb: 0.015%–0.030%, P≤0.01%, S≤0.01%, Ti≤0.021%, and Si+Al: 0.7%–1.5%, with the balance being iron and unavoidable impurities.

[0006] Furthermore, in the above technical solution, the plating solution of the hot-dip galvanized DH steel contains 0.16% to 0.25% Al by mass, with the remainder being Zn and unavoidable impurities; the zinc layer weight per unit area of ​​the hot-dip galvanized steel sheet is 60 to 200 g / m². 2 .

[0007] Furthermore, the microstructure of the hot-dip galvanized DH steel contains, by volume percentage, 35%–55% ferrite, 35%–50% martensite, 3%–12% retained austenite, and 3%–15% bainite.

[0008] Furthermore, the hot-dip galvanized DH steel has a yield strength of 420–540 MPa and a tensile strength of 780–870 MPa along the direction perpendicular to the rolling direction. 80 Elongation after fracture is 20%–24%, thickness is 1.0–1.6 mm, and hole expansion rate is ≥25%. The steel plate surface rating reaches Grade C.

[0009] A method for manufacturing the aforementioned 780MPa grade hot-dip galvanized DH steel, the method comprising the steps of converter smelting, slab continuous casting, hot rolling, pickling and cold rolling, preheating, pre-oxidation, heating, slow cooling, rapid cooling, and galvanizing after rapid cooling; wherein...

[0010] Hot rolling: billet loading temperature 450℃~650℃, heating temperature 1200℃~1300℃, initial rolling temperature 1050℃~1150℃, final rolling temperature 855℃~900℃, coiling temperature 620℃~650℃; descaling water pressure 16~18MPa.

[0011] Pickling and cold rolling: Pickling speed 100~150m / s, pickling temperature 75℃~85℃;

[0012] Galvanizing: The dew point in the heating section of the annealing furnace is -15℃ to -10℃, the dew point in the annealing and slow cooling sections is -40℃ to -30℃, and the dew point at the furnace nose is -50℃ to -40℃; the hydrogen (volume) content in the annealing furnace is 3% to 5%, the annealing temperature is 820℃ to 850℃, the slow cooling outlet temperature is 680℃ to 750℃, the rapid cooling rate is greater than 20℃ / s, the rapid cooling outlet temperature is 400℃ to 430℃, the galvanizing temperature is 450℃ to 460℃, and after galvanizing, the strip steel is first cooled to 400℃ to 420℃ by air knife; the finishing elongation rate during the finishing process is controlled within the range of 0.5% to 1.0%.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0014] This invention optimizes the alloy composition of DH780 steel, reducing the content of Si and Mn elements and decreasing the formation of external oxides, thereby reducing the risk of incomplete plating defects. Furthermore, by increasing the dew point during annealing, the oxygen partial pressure in the annealing atmosphere is increased, promoting internal oxidation of Si and Mn elements on the surface of DH780 steel, effectively reducing incomplete plating defects and significantly improving the surface quality of the steel sheet. This method not only reduces production costs but also improves the weldability and processability of the product, effectively enhancing the integrity and uniformity of the coating, improving the corrosion resistance and service life of the product, and broadening the application range of DH780 hot-dip galvanized steel sheets. Attached Figure Description

[0015] Figure 1 The image shows the microstructure of the 780MPa grade hot-dip galvanized DH steel prepared in Example 1.

[0016] Figure 2 Tensile curve of 780MPa grade hot-dip galvanized DH steel prepared in Example 1;

[0017] Figure 3 The image shows the surface of the 780MPa grade hot-dip galvanized DH steel sheet prepared in Example 1. Detailed Implementation

[0018] The present invention will be further described below with reference to specific embodiments, but this does not limit the invention in any way. To avoid redundancy, unless otherwise specified, the raw materials used in the following embodiments are all commercially available products, and the methods used are all conventional methods unless otherwise specified.

[0019] A 780MPa grade hot-dip galvanized DH steel, wherein the chemical composition of the hot-dip galvanized DH steel is as follows (mass content): C: 0.15%–0.18%, Mn: 1.6%–2.0%, Si: 0.2%–0.6%, Cr: 0.10%–0.15%, Al: 0.4%–1.0%, Nb: 0.015%–0.030%, P≤0.01%, S≤0.01%, Ti≤0.021%, and Si+Al: 0.7%–1.5%, with the balance being iron and unavoidable impurities.

[0020] A method for manufacturing the aforementioned 780MPa grade hot-dip galvanized DH steel includes the following steps: converter smelting, slab continuous casting, hot rolling, pickling and cold rolling, preheating, pre-oxidation, heating, slow cooling, rapid cooling, and galvanizing after rapid cooling.

[0021] The hot-dip galvanized DH steel bath contains 0.16% to 0.25% Al by mass, with the remainder being Zn and unavoidable impurities; the zinc layer weight per unit area of ​​the hot-dip galvanized steel sheet is 60 to 200 g / m². 2 .

[0022] Any aspects not described in the following embodiments are the same as those described in the specific embodiments above.

[0023] Example

[0024] A 780MPa grade hot-dip galvanized DH steel, the specific chemical composition of the hot-dip galvanized DH steel described in Examples 1-10 is shown in Table 1.

[0025] Table 1. Chemical composition (wt%) of hot-dip galvanized DH steel in the examples.

[0026]

[0027]

[0028] A method for manufacturing 780MPa grade hot-dip galvanized DH steel, the specific process flow of Examples 1-10 is as follows, Table 2 shows the hot rolling process parameters; billet furnace temperature 450℃~650℃, heating temperature 1200℃~1300℃, initial rolling temperature 1050℃~1150℃, final rolling temperature 855℃~900℃, coiling temperature 620℃~650℃; descaling water pressure 16~18MPa.

[0029] Table 2 Hot rolling process parameters of the manufacturing method in the embodiments

[0030]

[0031] Table 3 lists the process parameters for cold rolling and hot-dip galvanizing of the steel in the examples. For pickling and cold rolling: pickling speed 100–150 m / s, pickling temperature 75°C–85°C. For galvanizing: dew point in the annealing furnace -15°C–-10°C, hydrogen (volume) content in the heating and annealing sections 3%–5%, dew point at the furnace nose -50°C–-40°C; annealing temperature 820°C–850°C, slow cooling outlet temperature 680°C–750°C, rapid cooling rate greater than 20°C / s, rapid cooling outlet temperature 400°C–430°C, galvanizing temperature 450°C–460°C, after galvanizing, the strip is first cooled to 400°C–420°C by air knife; the finishing elongation during the finishing process is controlled within the range of 0.5%–1.0%.

[0032] Table 3. Cold rolling and hot-dip galvanizing process parameters for the manufacturing methods of the embodiments.

[0033]

[0034] The mechanical properties and key parameters of the 780MPa grade hot-dip galvanized DH steel obtained in Examples 1-10 are shown in Table 4. The yield strength of the hot-dip galvanized DH steel perpendicular to the rolling direction is 420-540MPa, and the tensile strength is 780-870MPa. 80The elongation after fracture is 20%–24%, the thickness is 1.0–1.6 mm, and the porosity is ≥25%. The microstructure morphology of the 780 MPa grade hot-dip galvanized DH steel prepared in Example 1 is shown in the image below. Figure 1 As shown; stretch curve, as Figure 2 As shown; the surface condition of the steel plate is as follows Figure 3 As shown, the steel plate surface rating reaches grade C.

[0035] Table 4. Performance parameters of 780MPa grade hot-dip galvanized DH steel prepared in the examples.

[0036] Example Thickness / mm Yield strength / MPa Tensile strength / MPa <![CDATA[Elongation A 80 > Hole expansion rate % 1 1.2 507 795 24 25 2 1 526 829 24 27 3 1.4 461 802 20 29 4 1.6 502 801 22 29 5 1.1 447 815 23 25 6 1.5 470 832 23 25 7 1.4 496 802 21 25 8 1 496 780 20 26 9 1.1 517 829 21 28 10 1 466 862 21 28

[0037] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A 780MPa grade hot-dip galvanized DH steel, characterized in that, The hot-dip galvanized DH steel has the following chemical composition by mass: C: 0.15%~0.169%, Mn: 1.6%~2.0%, Si: 0.2%~0.5%, Cr: 0.10%~0.15%, Al: 0.4%~0.9%, Nb: 0.015%~0.030%, P≤0.01%, S≤0.01%, Ti≤0.021%, and Si+Al: 0.7%~1.5%, with the balance being iron and unavoidable impurities; The hot-dip galvanized DH steel has a plating solution containing 0.16% to 0.25% Al, with the remainder being Zn and unavoidable impurities. The zinc coating weight per unit area of ​​hot-dip galvanized steel sheet is 60~200g / m². 2 ; The microstructure of the hot-dip galvanized DH steel contains 35%~55% ferrite, 35%~50% martensite, 3%~12% retained austenite, and 3%~15% bainite by volume percentage. The hot-dip galvanized DH steel has a yield strength of 420~540MPa and a tensile strength of 780~870MPa along the direction perpendicular to the rolling. 80 Elongation after fracture 20%~24%, thickness 1.0~1.6mm, and porosity ≥25%; The manufacturing method of the 780MPa grade hot-dip galvanized DH steel includes the following steps: converter smelting, slab continuous casting, hot rolling, pickling and cold rolling, preheating, pre-oxidation, heating, slow cooling, rapid cooling, and galvanizing. in, Hot rolling: billet loading temperature 450℃~650℃, heating temperature 1200℃~1300℃, initial rolling temperature 1050℃~1150℃, final rolling temperature 855℃~900℃, coiling temperature 620℃~650℃; descaling water pressure 16~18MPa. Pickling and cold rolling: Pickling speed 100~150m / s, pickling temperature 75℃~85℃; Galvanizing: The dew point in the heating section of the annealing furnace is -15℃ to -10℃, the dew point in the annealing and slow cooling sections is -40℃ to -30℃, and the dew point in the furnace nose is -50℃ to -40℃; the hydrogen content in the annealing furnace is 3% to 5%; the annealing temperature is 820℃ to 850℃, the slow cooling outlet temperature is 680℃ to 750℃, the rapid cooling rate is greater than 20℃ / s, the rapid cooling outlet temperature is 400℃ to 430℃, the galvanizing temperature is 450℃ to 460℃, and the strip is cooled to 400℃ to 420℃ by the air knife after galvanizing; the finishing elongation rate during the finishing process is controlled within the range of 0.5% to 1.0%.

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