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

Abstract

The invention discloses 780MPa-grade hot-dip galvanized DH steel and a manufacturing method thereof, the alloy components of DH780 steel grade are optimized, the contents of Si and Mn elements are reduced, and the generation of external oxides is reduced, so that the risk of plating leakage defects is reduced; besides, by improving the dew point in the annealing process and increasing the oxygen partial pressure in the annealing atmosphere, internal oxidation of Si and Mn elements on the surface layer of the DH780 steel grade is promoted, so that the plating leakage defect is effectively reduced, and the surface quality of the steel plate is remarkably improved; according to the method, the production cost is reduced, the weldability and machinability of a product are improved, the integrity and uniformity of a plating layer are effectively improved, the corrosion resistance of the product is improved, the service life of the product is prolonged, and the application range of the DH780 steel grade hot-dip galvanized steel plate is widened.

Description

Technical Field

[0001] The invention belongs to the technical field of high-strength steel, and specifically relates to a 780MPa grade hot-dip galvanized DH steel and a manufacturing method thereof. Background Art

[0002] In the production process of hot-dip galvanized steel sheets, the surface quality of DH780 steel, as a high-strength and high-toughness steel, is a key indicator to measure the quality of the product. However, the missing plating defect has always been a common problem affecting the surface quality of DH780 steel hot-dip galvanized steel sheets; missing plating not only damages the appearance of the product, but also reduces its corrosion resistance and service life, and increases the subsequent maintenance and replacement costs.

[0003] In the prior art, the effect of reducing external oxidation and plating leakage by reducing the content of Si and Mn is limited, because Si and Mn elements play an important role in improving the strength and other properties of steel, and excessively reducing their content may damage the overall performance of steel. For example, the patent "A 590MPa-level low-cost, high-surface quality hot-dip galvanized high-strength steel and its production method" with application number CN202311146689.4, its pre-oxidation treatment can improve the wettability and adhesion of the steel surface, and improve the uniformity and integrity of the coating; but relying solely on pre-oxidation treatment cannot completely solve the problem of plating leakage caused by external oxidation of Si and Mn elements. Summary of the invention

[0004] In order to overcome the defects of the above-mentioned prior art, the purpose of the present invention is to reduce the content of Si and Mn elements and the generation of external oxides by optimizing the alloy composition of DH780 steel, thereby reducing the risk of missed plating defects; in addition, by increasing the dew point during annealing, the oxygen partial pressure in the annealing atmosphere is increased, which promotes the internal oxidation of Si and Mn elements on the surface of DH780 steel, thereby effectively reducing the occurrence of missed plating defects and significantly improving the surface quality of the steel plate; this method not only reduces the production cost, but also helps to improve the weldability and processability of the product, effectively improves the integrity and uniformity of the coating, improves the corrosion resistance and service life of the product, and broadens the application range of DH780 steel hot-dip galvanized steel sheet.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides a 780MPa grade hot-dip galvanized DH steel, wherein the chemical composition mass content of the hot-dip galvanized DH steel is 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%, and the balance is iron and unavoidable impurities.

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

[0007] Furthermore, the microstructure of the hot-dip galvanized DH steel contains, by volume percentage, 35% to 55% ferrite, 35% to 50% martensite, 3% to 12% retained austenite, and 3% to 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 perpendicular to the rolling direction. 80 The elongation after fracture is 20% to 24%, the thickness is 1.0 to 1.6 mm, the hole expansion rate is ≥ 25%. The surface rating of the steel plate reaches C grade.

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

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

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

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

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] The present invention optimizes the alloy composition of DH780 steel, reduces the content of Si and Mn elements, reduces the generation of external oxides, and thus reduces the risk of missed plating defects; in addition, by increasing the dew point during annealing, the oxygen partial pressure in the annealing atmosphere is increased, and internal oxidation of Si and Mn elements on the surface of DH780 steel is promoted, thereby effectively reducing the occurrence of missed plating defects and significantly improving the surface quality of the steel plate; the method not only reduces the production cost, but also helps to improve the weldability and processability of the product, effectively improves the integrity and uniformity of the coating, improves the corrosion resistance and service life of the product, and broadens the application range of DH780 steel hot-dip galvanized steel sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The microstructure morphology of the 780MPa grade hot-dip galvanized DH steel obtained in Example 1;

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

[0017] Figure 3 This is a surface diagram of a steel plate of 780MPa grade hot-dip galvanized DH steel prepared in Example 1. DETAILED DESCRIPTION

[0018] The present invention is further described below in conjunction with specific examples, but the present invention is not limited in any way. To avoid redundant description, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0019] A 780MPa grade hot-dip galvanized DH steel, wherein the chemical composition mass content of the hot-dip galvanized DH steel is 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%, and the balance is iron and unavoidable impurities.

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

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

[0022] The matters not described in the following embodiments are the same as the description of the above specific implementation methods.

[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 of hot-dip galvanized DH steel in the embodiment (wt%)

[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, and Table 2 is the hot rolling process parameters; the billet entering furnace temperature is 450°C-650°C, the heating temperature is 1200°C-1300°C, the starting rolling temperature is 1050°C-1150°C, the final rolling temperature is 855°C-900°C, and the coiling temperature is 620°C-650°C; the descaling water pressure is 16-18MPa.

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

[0030]

[0031] Table 3 lists the process parameters of cold rolling and hot-dip galvanizing of the example steel. Pickling cold rolling: pickling speed 100-150 m / s, pickling temperature 75°C-85°C. Galvanizing: the dew point in the annealing furnace is -15°C-10°C, the hydrogen (volume) content in the heating and annealing sections is 3%-5%, and the dew point of the furnace nose is -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 skin-passing elongation of the skin-passing process is controlled within the range of 0.5%-1.0%.

[0032] Table 3 Cold rolling and hot dip galvanizing process parameters of the manufacturing method of the embodiment

[0033]

[0034] The mechanical properties and key parameters of the 780MPa grade hot-dip galvanized DH steel obtained in Example 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, the tensile strength is 780-870MPa, and the A 80The elongation after fracture is 20% to 24%, the thickness is 1.0 to 1.6 mm, and the hole expansion rate is ≥ 25%. The microstructure morphology of the 780MPa grade hot-dip galvanized DH steel obtained in Example 1 is as follows: Figure 1 As shown; the stretching curve diagram, such as Figure 2 As shown; the surface condition of the steel plate is as 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 obtained in Example

[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 any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A 780 MPa grade hot-dip galvanized DH steel, characterized in that: The chemical composition mass content of the hot-dip galvanized DH steel is 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%, and the balance is iron and unavoidable impurities.

2. The hot-dip galvanized DH steel according to claim 1, characterized in that: The plating solution of the hot-dip galvanized DH steel contains 0.16% to 0.25% Al, and the rest is Zn and unavoidable impurities; the weight of the zinc layer per unit area of ​​the hot-dip galvanized steel sheet is 60 to 200 g / m 2 .

3. The hot-dip galvanized DH steel according to claim 1, characterized in that: The microstructure of the hot-dip galvanized DH steel contains 35% to 55% ferrite, 35% to 50% martensite, 3% to 12% residual austenite, and 3% to 15% bainite by volume percentage.

4. The hot-dip galvanized DH steel according to claim 1, characterized in that: The hot-dip galvanized DH steel has a yield strength of 420-540 MPa and a tensile strength of 780-870 MPa perpendicular to the rolling direction. 80 The elongation after fracture is 20% to 24%, the thickness is 1.0 to 1.6 mm, and the hole expansion rate is ≥25%.

5. A method for manufacturing 780 MPa grade hot-dip galvanized DH steel as claimed in claim 1, characterized in that: The manufacturing method comprises the following steps: converter smelting, slab continuous casting, hot rolling, pickling and cold rolling, preheating, preoxidation, heating, slow cooling, fast cooling, and galvanizing; in, Hot rolling: billet furnace temperature 450℃~650℃, heating temperature 1200℃~1300℃, rolling start temperature 1050℃~1150℃, final rolling temperature 855℃~900℃, coiling temperature 620℃~650℃; descaling water pressure 16~18 MPa; Pickling cold rolling: pickling speed 100-150 m / s, pickling temperature 75℃-85℃; Galvanizing: The dew point of the heating section in the annealing furnace is -15℃~-10℃, the dew point of the annealing section and slow cooling section is -40℃~-30℃, and the dew point of the furnace nose is -50℃~-40℃; the hydrogen content in the annealing furnace is 3%~5%. The annealing temperature is 820℃~850℃, the slow cooling outlet temperature is 680℃~750℃, the rapid cooling rate is greater than 20℃ / s, the rapid cooling outlet temperature is 400℃~430℃, the galvanizing temperature is 450℃~460℃, and the strip is first cooled to 400℃~420℃ by air knife after galvanizing; the skin-finishing elongation of the skin-finishing process is controlled within the range of 0.5%~1.0%.

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